There's also the problem of availability of unique status updates, the lower your character count the fewer unique status updates there will be. If you launch a service with only 1 character there will only be 25 possible status updates.

People want to be unique, but they're also lazy, so it's a compromise between the two. I think 14 characters strikes the balance.

25? Is that a typo or have I missed something?

Anyway, what's wrong with using numbers? Upper and lowercase? Unicode?