That's not exactly true. As rjk already pointed out, they have virtually no other use of significance outside of bitcoin if they're attached to the PCB's that we're using for mining (with the exception of Icarus). FPGAs weren't invented to mine bitcoins, they were just adapted for that purpose. Oddly enough, being adaptable for various purposes is *exactly* what FPGAs were designed for. Pop the spartan chip off of there and attach it to some other purpose built PCB with an appropriate bitstream loaded onto it and it's a completely different functional unit.
They may be useless outside of mining to *you*, but I guarantee there's a whole other industry out there who will find value in them for more than crunching hashes.