The following write-up by another LJ user (not someone I know, but their write-up was mentioned on a Browncoat mailing list I'm on) is not a bad summary of some of the reasoning behind Wash's fate in the movie:The one thing I'm not sure it really addresses is that ... in the real world, people get killed for no reason--no greater good. They get killed pointlessly. Even if you do think Wash's fate was pointless, one could say that this makes the movie even stronger, in that it is representing reality so much better than pretty much anything else out there.
Also beware that the comments on this post may well contain spoilers also, if there are any!
You have been warned.