I hate hardsubs. They screw with the picture a lot, unless you have really big files. I much prefer softsubs. AVI also tends to be lower quality, though obviously you can have a high quality file if you encode it right. But I don't care about file size so much, so I don't want re-encodes or anything that brings down quality. Since I play things from my laptop on vlc if I want to display on a tv, I also don't care about hardware compatibilities, though I have the software to convert to avi with hardsubs if absolutely necessary.
Since I can't stand errors or some of the more egregious styling, like different colors for different characters, I like to be able to change the styles, and avi just can't do that.
Beyond that, fansub groups are switching away from avi containers, so unless I want a crappy re-encoding, I can't get an avi file anyway.