Light vehicles are definitely getting some love on this thread - and they deserve it. Another of their uses is bombardment elements within expeditionary forces, especially for those not in rear echelon formations. While light vehicle bombardment loses some firepower-per size compared to infantry, their combination of good dodging and decent HPs makes them vastly more survivable than even armoured infantry.
Infantry, IMHO, are not getting enough love here. On the offense, infantry (especially when armoured) are my go-to elements on jungle, desert, mountain, and rift valley worlds, because they can be trained for these conditions, effectively giving them both superior firepower and supply efficiency, which in turn means transport space efficiency. On the defense, high entrenchment bonuses - especially if you've included construction elements - make foot truly difficult to dislodge.