Maybe to some extent the coat acts as an insulator against the heat, but not when the temps are extremely high, as we experienced where we used to live. You should have seen the transformation in my boy when he got his haircut -- a MUCH happier pooch! AFAIC, how the dog reacts is more important that what the "experts" say, and in my case, I had much expert advice encouraging me to have him clipped. Do note that I did NOT have him shaved... he was clipped short.
Note also that we did not have air-conditioning. There was no respite from the heat. Even the basement was hot. He did not use the kiddy pool after the first 2 summers. The beach was too far away for a quick dip in the lake. He HATES hoses. Nope... for us, clipping his coat short was the answer, and then avoiding being out in the sun as much as possible. We usually headed for the dog park around dusk, and any trips to the beach for a swim were in the late afternoon after the sun was past its peak.
My big dilemma, actually, was whether or not to allow him to swim, since this was so effective a means of cooling off, but tended to cause skin problems... But that's a thread of its own...
p.s. Don't you love the belly-up pose? I have a similar pic of Todd somewhere...