b/c they wick so well, they (due to their knit)often pull in moisture where you might not otherwise want it.  

I've had this experience where I had some moisture (not much)get in between my dry-top and tunnel and BAM!...instant wetness all over.  Think of how a lantern mantel works and it's very similar.  The technology wants to pull moisture away from you and disperse it over a large area (some manufacturers call it bi-component knitting).  This works great for sweat...but if you have a constant contact with moisture (even if it's not heavy amounts) the material will continue to pull in moisture from that source.  Make sense?

I've moved away from using "wicking" fabrics for paddling and have moved to IR thin skin.  I find it's comfy, warm and should it get wet, keeps some insulation qualities.

If you are going with base layers anyway, I'd recommend Marmot PreCip.  It's AMY technology works well against river funk.

That's my ten cents.