As for the OP, it seems as though all of the AOE tanks are seen as much squishier than the ranged, single-target tanks.
The melee companions do not have better AoE than the ranged ones, tanks or DPS, and I'm not even sure what give you such an idea. The ranged tanks are generally viewed in a better light because they do take less damage. Especially at the beginning of a fight, any melee enemy has to run up to start hitting a ranged tank whereas a melee tank starts taking it right in the face from the very beginning. They've also go lower comparative damage because they have to spend time running from target to target without getting anything in the way of an appreciable DPS, threat, or survivability increase to compensate.
Also, it's generally regarded that the ranged DPS companions have better AoE than the melee ones as well. The ranged companions get a specific AoE stance that lets them spam their AoEs at a much faster rate that that of the melee companions, giving them more out of that. Their single target stance also lets them deal ST damage at a better rate. Assuming you're willing to stance dance your companion based upon what you want him/her doing, you're better served with a ranged DPS companion than a melee one. If you don't really want to bother stance dancing your companion, just go with a melee DPS companion and turn on the only really useful stance (the DoT one).
As to the general lack of utility for tank companions, a lot of that comes from the fact that so much a PC tank's survivability depends upon spec, stance, and gear. You don't start getting the gear to be an effective tank until your mid-30s and, even then, unless you're providing the absolute best gear available for your tanking companion, his/her mitigation is still going to be down the tubes. The only thing the tank companions get is a stance that, really, is pretty friggin' terrible. It augments their threat gen even though their damage output is so low that you can pull threat by staring angrily at a target, but provides only 20% shield chance (compared to the 15% + spec + additional armor that the PC stances provide). The only reason that companion tanks are even remotely useful is because they spam the bejeebus out of their taunts. Without those taunts, they would never have aggro. As such, they're pretty much extremely low DPS characters with taunt buttons and generic level survivability. They've got threat, but no real survivability.