Resampling is a rather more complex operation than you might think
Well, I actually studied signal processing at university, so I have an idea of what it takes to resample a sound. (I actually did it with images once, but it wasn't very fast - processing a 2D signal is very complicated).
For sounds it's easier since you can have very good and still very fast methods. PA may be picking between perfect-slow and very fast-bad (And no, if I can guess what the 'trivial' method does, I wouldn't recommend it)
But I guess some programs already resample to 48000 internally, like mplayer (and maybe they can do it inside the MP3 decoding process, simplifying it considerably)
You can't rely on SIMD instructions always being available
Nowadays? Yes, you can. The machine I bought in 2002 (Athlon XP) has SIMD instructions. You can pretty much count on any machine today having SSE
Unless you're using a machine older than that in which case PA is the smallest of your problems.