Actually, the reason why most converters have a phpBB2 backend is because it can be converted to nearly every other platform in existence. Not having to worry about multitudes of other platforms reduces my maintenance workload significantly.
I still would like to, at some point, have pluggable front- and back-ends, so I don't have to deal with so many single-use converters. But for now, that's only a pipe dream.
Of course, if anyone is willing to take up the challenge and write such a framework, I'd be very happy