It is a way of strengthening the body and increasing mobility. It is an excellent form of exercise but it has become bastardised as quickly as it has become popular.
Also, the people who do it often have a very smug attitude about them as if they are doing the BEST exercise- much like yoga.
It is surprisingly difficult esp. for bodybuilders but its benefits are huge.
If you are gonna do it though do it 121 with an instructor who did at least 6 months full time training as well as yrs of personal practise. There are too many wkend workshop instructors out there.
BTW Patrick Swayze does Pilates!