One reason would be that you expect more than a few thousand users. More generally, for all but the very simplest apps I'm afraid can't agree with you (though,

One reason would be that you expect more than a few thousand users. More generally, for all but the very simplest apps I'm afraid can't agree with you (though, to clarify, this isn't really addressing the topic of the original post). Years ago I had a summer job maintaing a set of Perl scripts that persisted online store inventories to flat files. It was horrible. I've never found Postgres to add much complexity to a project, but on the storage side it provides me with the reassurance that smart people have thought hard about issues of consistency, stability and performance -- which millions have tested -- and on the query side, I can do various clever things at amazing speed if ever that's required. Why would I not go for that?