Regarding TOW Humvee: 4-5 crewman are always better than 2:
SL - communication, coordination, spotting, building defensive/denial means
Gunner - well...
Engineer - for repairing the vehicles/ laying mines, fixing various denial structures
Medic - as the gunner (and the rest of the crew/near by infantry) gets often injured
HAT/LAT - maybe it's a bit redundant but redundancy can guarantee a successful kill in this situations
Besides this, the rest of the crew can ensure the safety of the TOW Humvee against enemy RPG, C4, etc, when is camping as well as contributing to capture the flag faster, defending a flag/position. Lets not forget that Humvees have good mobility, going places with a relative autonomy. Why not be all you can be and not just an rocket shooter? I would encourage a wider range of capabilities for this special squad.
Now regarding the Mechanized Infantry, on the most servers I've been playing, this squad has the right at ONE single armoured vehicle and it has priority. I was convinced that this is the situation on VG server as well. But I was wrong. I found that in a very unpleasant manner. Right now I feel that Mech Inf is treated like some sort of second hand squad that have to kiss the asses of the APC squad, begging for a vehicle.
On the side note: you never know when the APC squad is reclaiming the vehicle just because they lost theirs, instead waiting in line for the new one to spawn.
I would plead for having this simple (and common sense) rule for VG server. The efficiency of a mechanized infantry squad are so obvious that don't need to be mentioned here. Not to forget that most of the time, the APC squad member are forgetting what APC acronym stands for which is Armoured PERSONNEL CARRIER and not a tank.