Exactly. Work or "being too busy" is not a valid excuse to not use your hard-earned benefits like PTO. Here's a newsflash - we're all busy. Schedule the time off well in advance and you'll 1) have something to look forward to and 2) have a deadline to work towards. I just put in my request last week to have 12/15-1/2 off.
Remember, work not getting done while you're out of the office isn't your problem -- it is the company's problem. The company isn't going to die because you take a couple of weeks off of work and if they WOULD die, well, they have bigger problems and you should probably look for another job.
(*and yes, if you own your own business or can cash out vacation days, I can understand not taking them)