The #1 reason is that you don’t need to do anything
ruby script/generate scaffold project name:string description:text estimated_hours:decimal
There. You’re done. No creating permissions, users, databases, etc. No need to run an extra process (mysql) just to try something out. The database is a file, so its portable. I use it almost exclusively for the first phases of my projects because of its ease of use and portability. I recommend it highly and am just thrilled by the fact that it’s the new default.
Some may complain that there aren’t many good GUI tools for SQLite… but with migrations, tests, and script/console, that’s not an issue at all.
That’s my .02