Is the save data for Angry Birds still there even if the app isn't? Recently I've been playing around with the Cyanogen nightlies on my N1. Nightly cm7 1-8 were fine but then one day I lost the home button so I had to backup and then wipe and then eventually everything was fine. But after the wipe Angry Birds was gone and even after I restore I don't have Angry Birds installed. If I install it is the save data still there?

Side note, when you do a backup with one Titianium, does it backup the save data too? Is that only if you have root? How would a non-root user do a backup? And why is the save data not just a database file somewhere that is easy to backup? Is the integrity of scores at risk or something?
I believe you backed up from Recovery, correct?  If that is the case, then the backup should contain everything that was on your phone before the backup -- apps, saved games, etc.  So restoring the backup should have restored Angry Birds and its save data, as long as that was present before the backup.

Titanium can back up the app, the app's data, or both.  It does require root.  Non-root users have to deal with Recovery backups, which are not as flexible since you can't restore only bits and pieces -- it's all or nothing.  Of course, the SD card is easy to back up (plug it into a computer) and some apps may allow you to export data.

You'd have to ask Rovio why they chose not to use a database, but I'd guess it's because the save data is so simple.  If you open the file it's almost human-readable text.  Even easier to back up than a database.