I find it baffling mostly due to the fact that, for example, you could get ownership of even a light 'Mech or vehicle, sell it, and make more than you ever could with the Wealth trait. Or that the difficulty in increasing 'Mech size to Heavy from, say, Medium (shoot pilot / defeat 'Mech and repair it) is much quicker and probably easier than gaining XP in order to get better attributes.
However, I think there is another consideration to keep in mind: not everybody's a 'Mech pilot. What I mean by that is in a game with an infantryman, a medic, a civilian, and a 'MechWarrior, your challenges are probably going to consist largely of individual enemies or perhaps a lone military base--not an entire army. In which case, Vehicle becomes an INCREDIBLE trait to take, because it's simply worth so much more than any amount of weapon training or stealth training or wealth or all of the above combined. Being a 'Mech pilot (or even a tank pilot) on the personal scale is just a wonderful thing to have because any problem you can solve with a gun ( which IMHO is most problems ;) ) the 'Mech pilot can solve better.
What I've taken to doing is, in games where he PCs are part of a larger force, or where we're going to be very frequently switching to tactical-scale (as in, a Lance of soldiers fighting other Lances of 'Mechs or whatever), the Vehicle trait is free, or at least they get bonus XP toward it. My game right now is with a company-sized mercenary group built off the Field Manual Mercenaries rules. Rather than follow the RPG rules for vehicle ownership, I simply let them spend their Tech and Mass points on THEMSELVES in addition to the other 'Mech pilots in their group. Worked out rather well.
But yeah. I really do think it depends on the circumstances, and whether you're sticking to just AToW or AToW+Total War or what.