Well, the OS and bit width matters too. {x86-32, x86-64, 32-bit ARM, AArch64} on {Windows, Mac, Linux, Android, iOS} ends up describing a decently large set of targets.
Also, you'd be surprised how often people request minor platforms. It's enough that we regularly get people asking for a C backend because they only have a C compiler for that target (though I think a C backend ends up usually not being what those people actually want).
Hm... I probably don't know enough about compilers, but why does Windows/Mac/... matter for a compiler (I see why it would matter for the standard library)? AFAIK, the main differences between these platforms are calling conventions (and possibly exception mechanisms), which LLVM should abstract away.
You're missing at least (a) structure padding/alignment; (b) debug info; (c) name mangling. LLVM abstracts over some of the differences but definitely not all.
In any case, just having a Rust compiler isn't very useful; you need a standard library (or at least libcore) to do anything interesting with the language, and that's where most of the porting effort comes in.
Also, you'd be surprised how often people request minor platforms. It's enough that we regularly get people asking for a C backend because they only have a C compiler for that target (though I think a C backend ends up usually not being what those people actually want).