Well, I find the aspects help define your characters personality. So the seven, to me, is just kind of a natural progression through the characters life, I suppose.
The seven doesn't seem like too much. It's true that some don't get used but overall the good ones get used and the bad ones get changed at milestones for ones that are better. Over time, you have a good balance of aspects for different situations: combat, social, investigation/awareness...those being the big three I can think of - or at least I try to have aspects that help me in all those areas.
Personally, I wouldn't go with more than seven because there are also campaign aspects, scene aspects and temporary aspects that can be added on where appropriate.
Yeah, and what Santaphrax said.
EDIT:
The biggest challenge for me is coming up with aspects that encompass multiple dimensions of a phase. So if your character is 200 years old, his background aspect might have to cover a lot of ground. So sometimes it's tempting to want multiple aspects for a phase, but the key is to make the aspect broad enough encapsulate everything and still have it say what you want it to.
This may be why it's a challenge aspecting fictional characters.
Also consider that fictional character with lots of experience have changed. If you look at Harry's 7 aspects in the first book and compare them to the aspects he might have in the 13th, there will probably be significant Changes. If you're trying to put aspects on Harry that encompass all 13 books, it will be very difficult.
It might be easier to take a character and break them down into "X" chunks and make an aspect for each.