I've found that without a thermostat, the atf fluid will not fully heat up with commute driving and low temps. I don't think my temps got over 130F last time I logged it. It was in the 40's out (i think) and I was driving very old lady like. Again, this is not going to damage anything, but it will change the way the TCC engages. No TCC engagement means less power making it to the wheels. Once you start beating on the car, the temps will rise.
This all depends on how you're driving. Without an external cooler, it took back to back 3rd gear pulls in hot weather to get my atf higher than I was comfortable with. If you aren't beating on your car in hot weather, you may want to log some atf temps to see whether it's even worth it. Either way, the best option is to add a thermostat. Best of both worlds.