Default it boot in a 32 bit operating system, maybe to do not loose the persons that have allready bought a Mac and have a processor that
does not support 64 bit, so if you dont’t wanna have compatibility issues that default mode of booting may be fine,
there are also many application that are not allready ready for 64 bit, as Adode CS 4 , but same problem you may have on windows too
with visa or windows xp on 64 bit, but the difference between them it’s big.