People who are developing single-DB applications in OpenACS don't need to use query files at all for their custom code.
People who are developing multi-DB applications will want use query files.
The QD overhead, as Jeff suggests, appears to be very low when compared to the expense of executing a query. There's really nothing to be gained here performance-wise.
It does require some memory ... on OpenACS.org about 3MB IIRC (we looked at this when trying to figure out why our site was eating up so much memory). On a modern machine that's not a large number. And if we follow through and pull queries out of the Tcl files belonging to standard OpenACS project supported packages of course the compiled Tcl bytecode will shrink by a roughly comparable amount so the overall overhead is very low.
The single issue which I think all would agree is important is the start-up issue, as start-up time can be fairly slow on sites that use a large number of packages. It's not just query files, though - we're compiling every */tcl/*-[init/procs]*.tcl file on startup, too. It's not clear how much can be done about this.
I really don't see much value in a tool that folds .xql files back into .tcl files, to be honest. It's not something I'd ever use.
