My reasons:
-The idea of an anonymous, underground, hard-to-trace digital currency is sooo cyberpunk
-Bitcoin mining is the coolest thing ever
-I can't wait until Bitcoins get picked up in the mainstream media and it explodes in popularity
-I am fascinated by the math and crypto behind it all
-I envision bumper stickers in the future that say "Like my driving? Donate to 1fGkzEhyjtrfJ9EbBbkMgZuuRcSGT2rQ3!"
-I also envision cyber-hobos trolling the internet looking for spare bitcents