Apple's trackpads provide haptic feedback. If you push them gently, there is a soft click, if you push them more, there is a hard click.

An additional benefit over hard buttons is that applications can use it for additional feedback. E.g. OmniGraffle provides subtle haptic feedback via the trackpad when objects align, which I find extremely handy for quickly aligning objects.

I like distinct buttons. I like being able to tell by feel where my fingers are. I lose track of my finger's position on a big trackpad where the only clues to finger position are the edges of the pad. This leads to touch leakage: you go click but you touch halfway in the button area and half-way in the trackpad area and so the mouse pointer jumps as you click, thus clicking on the wrong thing. Talk about irritating!