Are you the kind of guy that gets paid on Friday and is broke on Monday???
I know someone who always had a lot of miles and gave them away like a drunken sailor, he never kept more than enough for a free trip or two in his account. After all, he was always travelling, so so what??
This guy has been out of work for awhile now and can't really afford to pay for the tickets he needs to get back on his feet. Guess what?? He really wishes that he still had those miles, as the people he gave them to long ago forgot his generosity.
As for me, I bank them all for one purpose. UPGRADES, I fly way too often and on flights way too long to stay in the back.
Also considering how cheap tickets are at the moment, make sure that your getting a good deal if you decide to use the miles.