do it easy:
What would happen if I put on my drysuit and dumped a gallon of water in it? Would that layer of trapped water help to keep me warmer?
For me, it's all about how the suit fits and letting in as little water as possible, not trapping water.
Apples and oranges. A dry suit isn't designed to be used with water on the inside and a wet suit is. A wet suit CAN'T keep the water out. I get that the wet suit is keeping the water and the diver warmer than without a wet suit, but that trapped layer is still warmer than the water on the outside and therefore plays a role once it is trapped.
No matter how a wet suit fits, water will get in. Once it does, it is BETTER to trap that water, therefore that trapped water helps even if it is the great fitting wet suit that keeps it in there. It's there and it always will be. Now you just have to keep the NEW water from getting in.
Encyclopedia... "If water circulates freely in and out of the suit, however, the BODY uses ADDITIONAL energy to warm the NEW cold water. This is why snug fit is so important."
Scubadiving.com... "...you at least want to keep it from leaving quickly (BECAUSE IT'S NOW WARMER THAN THE WATER ON THE OUTSIDE EVEN IF IT DID ORIGINALLY AND STILL CONTINUES TO ROB HEAT), so "trapping" is a good thing."
It is all about keeping the water out, I agree, but when using a wet suit you really can't keep it out so NOW you HAVE to trap it. You have no other choice. If you don't trap it, you get colder faster. Fact.