eq1 added companions which was good as some classes cant solo and they can be used as a group member. sadly content was changed to be harder such that using one is a must for even for the soloable classes. there are various tiers of strength. after level 80 you MUST use the SUBSRCIPTION only ones.
swtor has them but you had to unlock them as you progress through the story line. you also had to equip them which was problematic. they can take the place of a group member. they could go out and sell for you while soloing. the healers kinda sux as after a heal all of the unattacked mobs in the group would start beating on it and then it would heal itself and not you. the tank ones were ok except for the wookie which had a aoe knockback. so after using that in a tight area like a city you can and would have now several groups on you. death by wookie muhahahahaha.
eq2 added them which helped some classes solo more easier. they could take the place of group members. the latest one I had was the orc which was too powerful as it was almost impossible to kill unless you played stupid. it was a battle healer and low levels would pl themselves in certain instant dungeons. this got nerfed. it would rez you or group members too err when it wanted to lol.
companion really are not needed in this game as the pet ai is just terrible, solo content is really easy and there are graphic lag issues in several places.
besides if implemented I can guarantee that you MUST have a SUBSCRIPTION to use the best ones as eq1 and eq2 have done.