now i have not played just a whole bunch of mmos, but in all of the ones that i have played the pets/summons system has been nearly if not exactly the same. you choose your tree or class or whatever that summons and on that tree/class as you level up you unlock two or three different pets and can only have one out at a time. this is not exactly what i had in mind for what a summoner would do. it seems limited, unimaginative, and lame.
when i imagined a "necromancer" in an rpg i imagined someone who would summon 50 skeletons, for a battle, weak but en masse can balance out someone at a similar level. rift provides the perfect resource for the summoning because of the different planes. why not have entire trees for the summoning classes that consist of starting off with one or two weaker things and then either choosing to go the way of one or two powerful summons or a horde of weaker ones; and even leave open the possibility of mixing the two. 30 zombies and a demon. 75 rats and some sort of huge wyrm or beast. maybe the summons dont stick around with you for extended periods between battles or fights but you can have a set list of summons that that individual summoner has prepared and then a limit to the amount of creatures he can support at once.
have a different summoning tree for each plane. you could mix between a plane or two if you wish but you would never reach to highest summons of either plane but could have the opportunities for different combos.
honestly i just want to see a summoner summon up an army and have a warrior go wading through the ranks sending them flying. he takes damage because they can surround him and get in a hit or two or few before he dispatches them depending on their prowess. i dont know much about pvp but im sure there are ways that summoners would find their places in it. they would be awfully distracting at any rate and if left unchecked could be the enemy's downfall.
i have been looking for an mmo with this type of mechanic for a while. i dont know if anyone is just too scared to tackle such a thing or what but no one has done it as far as i know and i really wish someone would.
if controlling the summons is a question, look to strategy games like the total war's battles. they provide a near perfect system for controlling large groups.