what's the reason for different front and rear tire sizes on a XC MTB? i just received my Moots Mooto X RSL, which i had previously ordered--it's a titanium 29" hardtail, spec'd for XC racing (eg, an upright geometry w/ 100 mm travel on ff, fairly high head angle &amp; seat angle) 

Moots (AFAIK) only builds to-order but i know next to nothing about mtb wheels and tires so i deferred to their default spec for both

i just picked up the next bike from the dealer yesterday and the front has a 2.4" wheel; the rear a 2.2"

is this common on mtb builds? Or at least for mtb XC racing builds?

what is the reasoning behind this practice (i assume the wider wheel in front is critical)?
Yes, it is common on mountain bikes to have a wider front tire.  This is to avoid "washing out" in corners.  Plus the extra air volume provides more cushion and increased comfort for your hands (even if you have suspension).