If you really believe your setup is so much better than what 100s of other sysadmins are doing around the world, then good for you.
it is
Well, it really isnt because it isnt linearly scalable. So maybe for your specific setup it works, but you're in a zone of users thats probably kind of small. The bulk is the group that just wants RC to work out of the box. DB sessions provide that hassle free because you already need the DB. Another large group is the one of companies or ISPs or whatever that have multiple servers running, for redundancy and scalability. The group of admins that want 1 single server, running tmp filesystems on that for sessions, is probably really really small.
// Backend to use for session storage. Can either be 'db', 'memcache' or 'php' $rcmail_config['session_storage'] = 'php';
Roundcube is an open source project. You can always provide a patch for this and submit a pull request. I can see some situations in which a php session setup could be useful. Not many though. Probably not enough to bother coding for it. But if you do, maybe they'll add it.
Cor