Reasons I mine at Deepbit:
1. I'm lazy.
2. Contrary to the rest of my life, I picked what was the simplest solution at the time.
3. Their site had more features than any other pool when I signed up (still there because of point 1).
4. Mining teams, go BU.
I'm sure many of you would have some choice words for me, but it ultimately comes down to point 1 (that's why it's point 1). Deepbit was the simplest pool to use when I first started mining, and I can't be bothered to create a new account somewhere else / modify my shell scripts.
I will add though, if Deepbit were to gain 51% share (which it wont), I'd move out of it for the good of the currency (not that my 1.8GH/s would do much to Deepbits overall influence).