"Your margin is my opportunity." -- Jeff Bezos
It's interesting with a small portfolio (one vacation rental, for example) vs a larger portfolio (350 "keys"). If you have a few hundred keys, you can see how things are going and then start turning dials. So if you're behind, it's well worth it to give Expedia their 15-18%, because 85% of full revenue is better than 0%. This is why Expedia contracts are written to make it hard to cut them out if things are going well.
When you have 1-2 keys, though, once you fill, you fill. There's no more inventory to sell. You can't say "Oh, I'm pacing ahead, I can raise prices." It's already too late. If you think, "Their commission is too high," and you decide to wait it out and book without them, you might go empty.
So the decision about letting someone (Sykes, VRBO, whatever) turn your margin into their margin is a lot harder to manage as the season unfolds.