A friend of mine told me at spring, that he has read, that firmware 2.5 should have better video format support including separate subtitle files (srt etc.).
I think this is only rumour and even he didn't had any idea about possible mkv support, I don't see as an absolute impossible feature. Since we now have divx & xvid support, I can't see why we couldn't get mkv support too.
I don't think that adding better support for different video/audio formats to PS3 would increase piracy. It would just make things easier, when you shouldn't have any separate streaming boxes/multimedia pc's or shouldn't make any format conversions to watch your videofiles.
It would also decrease the willindness to mod your ps3, if you could get most of it even without any modifications. It would also make the PS3 more appealing for people who are more interested of watching videos than playing games.
A good video streaming box can easily cost 399,- € and in PS3 you also have a Blu-ray player included, DVD player with quite a good scaler, mp3/cd player and you can also play games/browse internet etc., even if your main interest is good quality video films.