A rationale given for gi training is that it slows down the action and forces you to use technique instead of explosive power. Would this apply if you were doing folkstyle/freestyle wrestling? Would people who wrestle or do other grappling arts that are not BJJ benefit from wearing the gi to make their technique tighter?
WHAT?
Uh....what? First of all thats not the rationale of the gi AT ALL. And second of all have you ever tried shooting a double while someone is holding your collar and/or sleeve? Just because its more difficult does not mean it helps your technique. 
Just noticed you said 'a' rationale and not 'the' rationale, my bad. But still I dont see how wearing a gi would help while wrestling (unless youre a masochist)
Nope, wrestling is a different animal. Concentrate more on hand control, under hooks, over hooks, head control. You can't grab clothes in wrestling unless you're looking to get DQ'd.
No wrestling is different and the collar grip makes wrestling a lot harder, and not in a productive way. 
joshjitsu - No wrestling is different and the collar grip makes wrestling a lot harder, and not in a productive way.
Put it way better than I did
I've thought about the fact that, aside from the standard Judo throws and wrestling takedowns, a lot of "pure BJJ Takedowns" are really wrestling with integrated gi grips...
For example, using Judo grips to create "kazushi" to get a Single or Double Leg is a combination of both a Judo technique and a wrestling technique, but that move can only used in BJJ and not in Judo or wrestling. (That's what I mean by a "pure BJJ takedown").
brixx139 -Uh....what? First of all thats not the rationale of the gi AT ALL. And second of all have you ever tried shooting a double while someone is holding your collar and/or sleeve? Just because its more difficult does not mean it helps your technique.
I've heard that rationale tossed around quite a bit, whether or not it is valid.
once you put on the gi for stand up....its judo grips all day.
Only thing I've noticed is that it does help with hand fighting. A good judo player will make you pay the first time you lose a gripping battle, so your hand fighting has to be precise.
Also if you want to develope judo throws and trips, I think its easier in gi. Once you develope some confidence in the core movements, you still have to translate them into a Greco gripping system.