"The term wicking is a marketing gimmick to me."
I partly agree, but more so i find it's misrepresented and misunderstood. It seems like companies market all synthetics as "wicking", which just isn't true, and personally i've noticed a big difference between a synthetic shirt that does wick well and one that doesn't in hot, humid weather.
You may (or probably) know the following, but sharing it for any newbies on the topic: Synthetics by nature, are not inherently wicking at all, but natural fibers are. Reason being is because synthetics are inherently hydrophobic. Synthetics have to be altered in some way, either by chemical treatment or special fiber structure or combination of different structure/size (like capilene, bi-component weaves).
The difference between between a wicking synthetic and a non wicking is essentially this. The "wicking" synthetic will immediately pull/seemingly absorb and spread out your sweat/moisture over a larger surface area increasing rate of evaporation which speeds up the cooling and drying of a garments fabric. In very humid and hot weather, this is crucial. This is why i liked my Echo LS shirt so much when i was recently in Costa Rica–it wicked very well, and combined with the fact that it was so thin and breathable (and it's polyester, so quicker drying depending on weave and thickness), made those conditions more tolerable. My linen shirt did really well when i wasn't too active (just walking around casually), but got too overwhelmed when active (hiking, walking up and down hills [of which there seem to be a lot of in C.R.], etc).
The non wicking synthetic, like that ubiquitous tight weaved nylon shirt mentioned originally, well the moisture just tends to get trapped and has to be moved more by air evaporation through the open interstices of fabric. It's not as efficient, and tends to trap more moisture to your skin. This, and that they are not that breathable, is why plenty of people have found these ubiquitous tight woven nylon shirts to be "clammy" feeling in hot, humid weather when active. You can tell the difference between a wicking and non wicking synthetic immediately by a quick test–just drop some water drops on the inside of the fabric. The wicking kind will immediately absorb and spread same, on the non wicking the water will just bead and sit on the fabric–though course under pressure, movement, etc it will speed up the moisture movement process some, and nylon seems to be only borderline hydrophobic compared to polyster and p.p.
It "may" be that in more extremely cold weather, a very hydrophobic, but non wicking fabric– say made out of polypropylene, might be an advantage, as there might be less cooling effect? Combine that with the fact that p.p. is really non conductive–also helps.
In hotter and drier weather, the strengths of a wicking synthetic is it's weakness. It dries way too fast. That's why cotton is pretty good for keeping you cool in very hot and dry conditions because it's a combo of conductive and absorbs AND holds moisture well–prolonging the evaporation effect, the combo of which keeps you cooler.
I'm speaking from both experience and holistic logic with all the above, i've been to the tropics, been to the desert, and to various in between climates (my wife is a travel whore!).