By any measure I'm no FPGA expert.  But from what I've learned it really depends on the board. Icarus seems to have possibilities beyond btc mining. But the issue really becomes one of expertise in FPGA design too.  BFL is a one-trick pony (apparently not reprogrammable except by BFL)
Same for X6500 (can be reprogrammed), not sure about ztex (can be reprogrammed).

 "easily" might be relative to your skills.

The goal is to recoup your expenditure by mining. Then once the board's paid for itself and turned a profit who cares if it's useful.
Well some of us do care and if you can use the board as a dev board you can learn something too.