When playing blitz, I think there is absolutely zero issue with playing for a win on the clock, even when you can't get one on the board. Just before I wrote this, I actually did that. Down by two points of material, I realized I had four minutes left and he had 2 and one half. I deliberately played into positions that I knew would take up time. I ended up with a checkmate this time, but only because he played some bad moves as his clock ticked down.
Blitz chess isn't "real" chess in my opinion. Blitz chess is a chess variant where the clock is an inherent, and important, part of the game. Using that tool is not just in the letter of the law, but the spirit as well, in my humble opinion.
In longer games, I might have a bigger objection to someone making bad chess moves from a lost position, just in the hope of running out the clock but, when all is said and done, it's perfectly legal and, while it might be annoying, it's no worse than that.
I didn't see the 50-move rule fully explained in this thread yet, so for the record: the 50-move rule states that a draw can be claimed (though it's not automatic) after 50 consecutive moves without a capture or a Pawn being moved.