I have no problem tipping 25% (I generally tip 20% or more). I make good money - definitely more than my servers do - and I count the tip as part of the cost of going out. When I was a server, it was not my "career" (it was during summers when I was in college and law school) but I appreciated being tipped well. I "needed" the money but it was not my only source of income (I had scholarships, grants, and student loans as well). I was never worried about whether I would be able to eat that week, but I know that it not the case for a lot of servers.
When I consider that other servers are in the position I was in (students trying to minimize loans and get a step up when they enter "the real world"), I want to tip them well. And then I consider that for some people server is their career and only source of income and I want to tip them even better. A few dollars more in a tip is immaterial to my bottom line (in fact, I factor it into my budget). However, a few dollars more from each patron could mean a single mother is able to pay her rent *and* school clothes this month, for example. I realize not everyone has the means to think this way, or the desire to think this way, but for me, I do not begrudge servers a few extra dollars out of my wallet.