I sometimes use a limit (constraint or varchar(n)) to enforce an upper bound when the domain doesn't really demand one, but you'd rather get an error that a longer string in your database. Say, 100 characters for a login name or 1000 characters for a real person's name.

Even if somebody actually shows up with a longer name, you know that your application will have other problems in that case (e.g. the UI explodes or something), but you prevent the occasional bug or other problem from writing megabytes into the name field, and causing follow-up problems.