While it's true that heavy use reduces the lifespan of a flash drive, most people rather underestimate how long they last. If you're considering using a flash drive for I/O-intensive tasks, I highly recommend Jeroen Kessel's description of the problem. He's specifically addressing defragmentation, but the second paragraph in particular applies to any kind of disk-heavy operation.
In particular, he says:
In order to wear out a cheap 10,000 cycle flash memory disk in ten years, you would have to write to EVERY BLOCK in the device about 2.7 times per day, every single day.
Minecraft's I/O usage went down dramatically in 1.3 (due to the switch to region files), but is still fairly high due to its frequent saving of chunks. Assuming a disk-wide average of 27 writes per block per day due to very heavy Minecraft play (due to wear leveling, that's about 108 gigabytes of written data for a 4GB drive), even an el-cheapo drive would last at least a year!