While I won't speculate either way on whether or not these are a good buy, pointing to the likely (much) higher future difficulty is only half of the equation.
A lot of people have already made an absolute killing on bitcoin by mining when others told them they were stupid because it wasn't profitable. A few years ago, when bitcoins cost a couple pennies each, people would run the same numbers and call those that chose to mine incapable of doing basic math.
Don't forget that a lot of people are mining today based on their own assumptions about the future price of cryptocurrency. Sure, the math doesn't look great based on ~$100 per BTC, but what is you assume bitcoin goes mainstream and hits $1000/BTC in a few years? There will only ever be 21 million coins, and it bitcoin becomes say, the internet currency of choice, then $1000 per BTC is quite a low estimate.
Those bad at math will still feel like having done a great deal then xD
