well, you would burn through your voting power pretty quick and not be able to upvote others, but that is currently your prerogative.

you're begging the question that everyone upvoting their own content will necessarily decrease quality, and I don't think the two are directly correlated.

Theres a reason "upvote your post" is an default option on new posts but not new comments -- because most people would do that anyways.

also, a quick example -- i've made a few comments in different threads today that I upvoted a small percentage just to push it to the top of the comment pile. Why? Because I want more people to see my comment. That's perfectly within my prerogative and my voting power reflects that.