I guess if you had to define what Pay To Fly actually is, dl_88 isn't far off the mark. Many people mistakenly think it's about the money.
If you've been in aviation for a while you've probably heard about the "swiss cheese" model. Basically what P2F is doing is looking (hunting - if you will) for the flimsiest stack of swiss cheese slices it can find, getting a trolley pole and shoving it smack down the middle of it, thus creating such a massive hole that any budding aviator, whether they have the aptitude or not can waltz into a position as Second in Command (note: responsibility) of a high performance aircraft that requires a lot more than a 200 total hour 'wonder of the skies' playing aeroplanes for real.
There has been a tried and tested way of recruiting experienced flight crew into such positions of responsibility. P2F by-passes the lot.
To make things worse, as they are for all intents and purposes buying a job, the worth of a pilot becomes less. You have airlines "employing" zero hour top guns and offering them ZERO salary for a certain time period, marketing it as "sponsored line training". What they're getting is a free pilot. Then what happens is other airlines see this. "Why should we PAY our pilots when our competitor is getting PAID?? This is fantastic!".
And so the P2F disease spreads. Experienced pilots at established airlines suddenly aren't getting pay increases, new employees are getting offered less and less each year. Airlines crying poor about "tough financial times". The people who participate in P2F schemes are laughing, they've just got 500 hours on type, time to move to the next job! Oh, wait, they only pay that much? What happened to the terms and conditions? Well, genious, you did this, to everyone. So, on behalf of the hard workers in the aviation industry, we thank you for contributing to its downfall.
Now please don't mistake, for example, someone who's been laid off already with 2000+ hours on type and are offered a job through an agency. These are experienced pilots. Again, it's not the money involved, it's the selfish risks these newbies take and are totally oblivious, nor care, about the implications of these actions. It's not just the agents who take advantage of these people either, the airlines must take the blame as well. But then again, as an airline why would you want to pay anyone anything, right?
To all those would-be P2F'ers - don't.