Is it safe to play the Macbook Pro/Air speakers at maximum volume?
The cones in almost any speaker will break down over time. Various environmental factors will determine how long a speaker lasts, but overdriving the element can definitely have an adverse effect.

It's impossible to provide any precise data but in short, you're probably shortening the life of the speakers (by an indeterminate amount of time) by running them at maximum volume but, provided you're not hearing any distortion from the speakers, you're not damaging them by doing so.