While I'm not really concerned about space, I really don't want 30MB file sizes per song since I do like to have a variety of songs on the iPod. I'm not too thrilled about 20MB for a 10 minutes song using 256kbit/s, but I can live with it since most my songs are 5 minutes and 10MB or less.
My main reason for the uniform bitrate is so all the songs sound the same, I don't want one track to be more quiet or sound worse then the next one.
And you're right, it's not the device that I need to worry about affecting quality, it's the headphones, speakers, etc. Right now, what I have is not the best, but when I get better equipment in the future I want my digital music to be ready for it.