Who’s Slowing down their Rebound when it gets wet and slippery?

I tend to keep settings the same and only adjust tire pressures or tires themselves when it's nasty. But I agree with you that if anything speeding up rebound makes the most sense due to the slower speeds. Especially since the temps are typically lower when the conditions are bad.

Maybe slowing rebound gives the feeling of more control by keeping the bike squatted and less lively feeling?

Great question! I think there's a simple suspension principle that applies here, that as rider speeds and trail speeds go up, rebound should get faster. As speeds go down, rebound should slow down as well. And I think that decision to speed up or slow rebound comes down to two things: packing and pushing.

1. Packing: as you noted, as trails dry out, speeds tend to go up. As a result, the rate of bumps hitting your wheels increases, as your suspension is hitting the same number of bumps in less time, and your fork is more likely to pack down and get trapped deep in the harsher/higher spring rate portion of travel if it isn't able to rebound to full length very quickly. Additionally, as trails dry out, holes tend to develop and get bigger in blown out conditions, which increases the demand on suspension. In addition to the now higher frequency of bumps, their intensity (total bump force) is also greater, making the downsides of suspension packing even more pronounced.

Here's a classic example of high speed dry conditions where suspension packing would be likely and where faster rebound speeds will likely help (an all-time favorite Vital RAW from the memory banks):

2. Pushing: This applies to front and rear suspension, but I think the wet weather/slow rebound thing is especially noticeable on a fork vs. a shock, because nobody likes their front tire slipping on wet roots, rocks, or mud. In the dry, when things are rough with big holes, there's effectively infinite grip as your wheels are dropping into holes, because your wheels are pinned into the hole and have no way to escape from the hole by slipping. In these sorts of dry and blown out conditions, you can run really fast rebound and it won't feel "scittery" from a traction perspective, where your suspension rebound force is unsettling the front tire and pushing you off line. "Scittery" is a highly technical term, btw. By contrast, if you imagine smooth wet rocks, smooth slick roots, or smooth slick mud, even a small increase in rebound damping (slowing it down) can make the contact patch on your front tire feel worlds more planted and controlled, and less likely to bounce off-line or push your front tire.

Example of slow speed slippery conditions that will benefit from slower rebound speeds:

Of course, there are exceptions to these principles, like the low-grip slippery "ball bearing" loose-over-hardpack conditions that often develop in the dry, especially in the flatter and smoother sections in between the holes. Or, conversely, when holes develop in the wet because of heavy rider traffic when a trail is saturated and the trail tread is unstable. But generally speaking, higher speeds benefit from faster rebound, and vice versa.

Now, to get back to your original question, I think there's something really important that you're hinting at, when you bring up lower shaft speeds in the wet. You said you "find this totally baffling. Wet conditions slow everything down therefore you will be using less stroke, have slower shaft speeds etc so a faster rebound setting should result in more traction. What am I missing here?"

Wet conditions do slow things down and produce lower shaft speeds on your suspension, but I think those lower trail speeds and lower shaft speeds should point a suspension tweaker toward reducing compression damping or even lowering spring rates, not increasing rebound speed. As trails get faster and rougher, it's often helpful to make suspension stiffer to absorb those greater forces, either by increasing compression, or spring rate, or both. The opposite is true in the wet. So I don't think your confusion is unfounded, I think you're just looking to change the wrong suspension parameter.

If you imagine starting from a platonic "base" suspension setting for a generic trail, in fast, rough, and dry conditions you would want to increase compression, open up rebound, and potentially increase spring rate. In wet, smooth, and slippery conditions you would want to open up compression, slow down rebound, and potentially decrease spring rate. But nobody does all that on an average trail ride, because that's way too much to think about. Adding a click or two of rebound on wet days as your friend does is a reasonable fascimile of the same philosophy.

Fortunately in the UK it's normally really cold when it's wet and relatively warm when it's dry so I feel like my rebound damping self adjusts with the temperature. Not sure how much the oil viscosity is actually changing, maybe the tyres also react differently in the cold also, certainly feels different though. 

I like the graph you shared and agree with what it's saying, but one thing to note about the graph is that it's only addressing changes in rebound speed. All other factors are held constant, i.e. rider weight, spring rate, compression damping, trail conditions, outside air temp, trail speed, riding style, etc. So, if you assume all of those other variables are constant, then this graph will help you understand how to view rebound setup. But as we respond to your original post, the hypothetical you're asking about specifically refers to changing trail conditions and trail speed, which means this whole graph and its curves will either shift to the left or to the right. The question we're debating is: which direction should we expect it to shift?

I expect that, as trail speeds drop and traction decreases, we would shift the curves of the graph to the right, towards slower rebound speeds, and vice versa for faster speeds and more grip.

I voted less rebound (so, faster) BUT when I live is colder when is wet, so suspension is going to respond slower anyway. But now that I think about it, if riding PROPER suspension, with good adjustable comp. damping dialled back up a bit for that slippery conditions... In that case I might end up slowing my rebound a bit. I always feel like I can get away with faster rebound the more compresion damping I get.

So yeah, for myself is a case of "tell me you are clueless about suspension without..."

This does not sound like a guy on vacation in NZ...😆

It's not really. Your oil will warm to operating temperature quite quickly, 30 seconds of moderate trail riding. Plus your more likely to notice the increase in compression damping when your suspension is cold than the slower rebound speed, which is normally the opposite of what you want in slow wet conditions. 

Jetlag is helluva drug... (maybe?)