Throw light jeeps and maybe unarmored Humvees in with the motorized troops, and you and I are pretty much on the same page. :thumbsup:
For me, the most important difference is not in mobility, but in protection.
Motor troops take full damage from antipersonnel guns, and very little from most antiarmor ones, implying light unarmored vehicles with no protection from small-caliber fire, while the unit as a whole is so dispersed that a PPC or AC burst will only kill one or two troopers.
On the other end of the spectrum, mechinf take less damage from burst-fire weapons, but more from the big guns. That says the bigger vees are armored to protect against small arms, but they still cannot protect against big guns, and a mech-scale hit will kill or disable a vehicle carrying more troops, hence the greater effect.