My 2 cents...
The value of MAC pentesting has to be measured against WHO you're working for, or WHERE you plan to pentest.
Consider the following two thoughts:
a.) You're pentesting for your current employer, and NO employees use MAC's, nor are there any production MAC's in place. There is lessened / little value in it, if this is your sole purpose.
b.) You're considering doing consulting work, and there's a good chance that at least some customers that you plan to work with ARE Mac shops (many publishers, newspapers, creative companies, educational institutions and others use a LOT of Mac.) In this case, there'd be heightened value.
I can also tell you, however, that MANY of the penetration testers I know HAVE Macbooks, and LOVE them, as their attack machines. Many of the tools exist for Mac's, as exist for Linux, etc, and a lot of these guys just like the Mac's interfaces and such, as a whole. Personally, I don't have one, but I'm certain I'd put it to use if I did, particularly if it was a Macbook, but even if it's just a desktop, as it never hurts to be well-rounded, and have that extra tool in your arsenal, even if just to test and learn from.
~ hayabusa ~
"All men can see these tactics whereby I conquer, but what none can see is the strategy out of which victory is evolved." - Sun Tzu, 'The Art of War'
OSCE, OSCP , GPEN, C|EH