I'll give out my personal opinion.
Am I glad I own both a PS3 and 360? Yes, I like both platforms a lot. However, I understand not everyone is going to feel the same way, and here's why.
I'm not even going to put blu-ray into this consideration.
-If you like/have interests in some of the Sony first party games out now and about to come out soon, then definitely buy a PS3 (I don't feel like making a list).
-If you have some interests in some (although PS3 doesn't have a lot) of third party exclusives like Ridge Racer 7, MGS4, Disgaea 3, Valkyria Chronicles, Naruto: Ultimate Ninja Storm, etc., then maybe buy a PS3.
-If you have interests in downloading games on PSN (and there are many great downloads on PSN now and many more great stuff in the future IMO), then definitely buy a PS3.
Those things IMO are the main things to differentiate the PS3 from the 360 (I'm assuming you want to keep 360 as your main platform). If you have interests in 2 or more of the three categories I listed, you should be perfectly fine with your PS3. If you are only interests in one category or none at all, I'd be cautious on buying a PS3.
Personally, PS3 is my main console and it gets all my multi-platform games. Also, I just really like Sony's first party games, PSN downloads, and even have interests in some third party games I can't get elsewhere (although there may not be many, since developing on PS3 is very expensive and a risk for most devs). I love my PS3 a lot and absolutely happy with it, but I understand if someone is heavily 360 and doesn't really care for much or any of the things from the categories I mentioned, they will probably be disappointed.
PS3 and 360 are similar and it CAN be tough for most to justify owning both, but in my view, I can easily find both worth owning (although not everyone will feel the same way).
Playing blu-ray movies is ONLY a bonus for me. If you badly want to play PS2 games, just buy a PS2 (Sony would be happier if that's what you did, since when they sold a PS3 with PS2 BC in some way, they lost money, unlike how they gain money on every PS2 sold). It's cheaper and if it's what you want, just buy that. If a firmware update messes with a game's compatibility, you don't have to worry about a PS2 because its results will never change. If you ever want to play a PS2 game that supports an accessory, just much easier to do on PS2. When PS3 has more games that maybe interests you more down the road, you can switch the PS2 to a PS3.