I've been meaning to create a spreadsheet for a while - should be easy to have the details for several different aircraft in there, and to just select the one you want each time. I'll post it when I've done it if anyone wants it.
I tend to agree though that this might lead to simply accepting the numbers given without thinking it through - so need to be very careful when using this method. Like any calculation, have a good idea of what the result should be before doing it.
One thing it would be useful for are problems where you need to work backwards without knowing the actual weights, e.g. If I'm fully fuelled, and I weigh X, what is the max weight I can carry in the back seat?" or "If I take mum and dad up, how much fuel can I carry?". This always seems a bit harder to calculate manually, but I'm just lazy.