I agree that it is customary, and higher-end restaurants are also typically higher in their attention to detail (or should be!), but I also see some merit in the other poster's question.
Let's just take Outback Steakhouse, for example. I can either order a kid's Joey sirloin, or the Alice Springs chicken, or the New York strip. They all come on the same sized plate, but due to the nature of the dish, they range in price from 6 bucks to 12 to 19. If I am a dutiful tipper, I would tip 2 to three times more for the steak dinner. Now I've fixed ALice Springs Chicken before, and personally, it's a lot harder than a steak on the grill! And even if it weren't -- well, it's the cook that did the slavin' not the server. So one could make the argument that the tip is therefore misplaced (i.e., better intended for the cook!) So I can see both sides of this "percentage" argument. And just how DID we Americans ever arrive at it being "Percent of bill" ? Was it just the easiest way to calculate it?
One interesting side note -- there are lots of countries that do not engage inthe practice of tipping. I know when I lived in Australia, I tried to tip at a restaurant and a local told me no way -- you're not starting that custom over here!
Personally, I am a somewhat generous tipper, my husband is more the "norm" (he's a 15% man, pretty much straight down the boards, good or bad. I've been known to tip from around 10% to 30 or 35, just depending upon the service.)