At my last job, I got to choose the balance between salary and equity I wanted -- and I really had to calculate if I wanted to earn a little less (or a lot less) in exchange for a greater share of future profits. Or to earn a little more (or a lot more) in exchange for giving that up. And having gained quite a bit of knowledge from the investor side of things, at least here in the tech industry in NYC, I don't think it's accurate at all to say it's "usually an exploitative relationship". At least, as long as employees bother to figure out how it all works.
Of course your employer can sack you or withold a raise. But of course you can leave for another company, or tell them you're leaving if they don't give you a raise.
But you've got to have enough skill to be of value, and enough negotiating skill not to be taken advantage of, as well. Just like a company has to have enough skill not to hire not to be taken advantage of by its employees -- the employees who don't contribute, the employees who spend more time playing politics, etc.
