The theory is good with the new buttonless MacBook trackpads, but in practice I think it's flawed. The whole very large surface clicks down when pushed, though it's hinged at the far side, so it works best at the side closest to the user. So you ought to be able to use your thumb to click exactly where the button used to be, and be no worse off than before. Problem is, the button was raised up a bit above the palmrest, whereas the tracking surface is a shade below it. So now your thumb has to reach down and over the lip to click, which is a bit uncomfortable. It's a subtle difference but I think an important one.
I've only had a play in the shop, so maybe I'd get used to it and maybe even change my clicking habits entirely, but it put me right off.