Hi !!
aside from that, i think it would be a useful addition that would bring more users to roundcube. i can understand not choosing to use a file based mechanism, but i'm not sure i understand not supporting one (provided its implementation is ok).
It depends on how you implement it. Now the whole data reading/writing is based on SQL queries. If you have a nice way for handling both DB and file based, please post the code and we will decide.
what about sqlite ? even better, what about using PEAR:DB ? PEAR:DB supports mysql, postgres, sqlite, dbase and odbc, so file based db's could be easely used.