Hi, you must be new to MMORPGs.
http://camelotherald.wikia.com/wiki/Necromancer
http://wiki.ffxiclopedia.org/wiki/Summoner
It's been done before. It doesn't work out as well as you think it would because:
* If the pet dies, you lose all your damage, utility, etc.
* You're partially at the mercy ofthe pet's AI. There's been a few cases where a player will's command will be issued, but simply lost in the queue. Oh yeah, pet pathing generally sucks.
* Stats are a nightmare. If the game snaphots your stats when you summon, you can frontload and suddenly your pet is unstoppable. Or, your stats do nothing in which case your gear is irrelevant. (This one, admittedly isn't as much of a problem, since a game can simply use the player's stats for calculating damage).
* Tethering. If the pet gets tethered, but the the game can't generate a path to you, it will probably despawn. Opps.
* Healing - healers now have to pay attention to your pet as well as you, since your pet is damage. Sure, you have may have a healing ability as well, but people have a tendency to tunnel at the worst possible moments and suddenly, the pet is dead.
The most successful pet classes have always had a portion of the player's damage come from the player.