I like Mozilla as a web browser, but it's really bad at everything else. That, and it completely goes against the unix philosophy.
Basically, I think of it like this: If you stuff an IRC client, email & news readers, address book, and a web browser into one program, it gets all bloated and slow. I recompiled mozilla to be nothing but a web browser so it would be faster.
Also, I happen to think that mutt makes a better mail client than mozilla, so having a mail client in mozilla would just be useless and redundant (especially considering it's not as robust).