but it is possible to run large boards on a flat-file based system without many problems

umm... no. flat file based stuff is not scalable because most (all?) of the software using this kind of db, does not implement a sophisticated filesystem. Also there is a possibility to reach a # of files in a directory limit in shared hostings ith too many members etc. A database removes these kind of problems. Even if you have your own vps/machine and use a flat file system on a very busy site, you'll experience huge slowdowns related to too much disk access. Even DB_File will be better than a hand crafted flat file db.

in many cases database backed systems perform better

I think it's better in every case unless you have only 10 users.