The gearing can be a bit odd, but I think in some cases they did it to make it simpler. For example - Elara Dorne using Aim instead of Cunning. I guess they decided that since she's a "Trooper" she should wear Heavy Armor. So they kept Aim as her primary stat so you wouldn't then have to have a bunch of Cunning Heavy Armor. However, her gun has Cunning on it insead of Aim, which suggests that they initially developer her as Cunning and switched part way through.
But overall that may be part of the explanation - they wanted to an overall look and feel to the companions, and they didn't want to then come up with bizaare gear that would be useless for everyone else.
All the melee tanks using Aim may be for the same reason - they felt that overall it would be easier for people to gear them that way. Rumor has it tha Qyzek was originally designed as an ranged companion as well.
Well not all the Tanks use Aim.. but yes most of them. But it would only make sense if they were Melee then they should use Strength. and if Range Aim. but maybe that's just me. lol