At my last job, I was being paid at a mid-level admin rate but was expected to do senior-level engineering and even architectural work (I had proven that I could). The best they could do is 3-10% (and even that was pulling teeth) per year. I had already been there several years and it would have taken me several more to finally make what I should have been making all along... minus the opportunity for raises based on that income.
In the end: I went out, interviewed, and negotiated a salary elsewhere at such an insanely high increase (just under 40%) that I figured my then-current employer would just show me the door and say thanks for my time. Much to my surprise, despite having a hard time squeaking out a few extra K before, they were not only offering to match my offer but exceed it by an appreciable margin.
The moral of the story: a lot of employers will aim to profit on your complacency. Period. It is more profitable to pay you the least as possible as long as possible. They got damn good (if I do say so myself) engineer at a steep discount. Even if I had taken their offer (which I didn't), it would have taken me a couple more years just for my pay to normalize to what it should have been all along.