1. You certainly don't need to write in assembly to call AltiVec when programming on a Macintosh. Apple makes it VERY easy to code.

2. As a real life scientist, I use single precision numbers all the time in my code, with the number of significant figures available from my data sets it's all I need. I suspect the same is true of many other scientists working on real world data sets, as most instrumentation hardly has the precision of a single, yet alone a double.