Well, I have a Persian in my house, and we actually get her shaved in the summer so her fur is short and she doesn't overheat. It is humane, the fact remains whether or not you do it yourself or get it done by a professional.
The Persians thick coat works as a insulator against heat as well as cold. I'm sure it is simmilar with other long haired cats. And random is right... if you brush/comb them out they will lose a lot of their winter coat and have a much thinner coat. Bathing will take out the winter coat too and there are a lot of tips on my website that make it much easier on you and the cat. They get used to it. Mine did after the 1st time and I got my cats all as adults! Two of them from the humane society so I diddn't know if they had been exposed to bath at all! Personally, I don't like shaving persian's. Temps get over 100 here in the summer and my cats tolerate it very well. They don't show any signs of being over heated anymore then a shorter haired cat. Remember... shavers are noisey, they vibrate and pull, and not to mention the cat looks funny! Poor things!