Your poll seems a little broken; it currently says 'M' or 'No!'.
Anyway, I imagine the summoners are quite hard to program and balance. You have to program in pet commands to control your minions, which is annoying if it's just for one class. You also have to program in friendly buffs that can land on AI monsters, but not enemy AI monsters and sometimes not AI monsters summoned by other people. More lines to write, more tags to check every time a spell is cast. Then you have to have a graphics engine that is capable of supporting all those little imps and goblins, quite a big ask for a free to play game. I mentioned balance earlier too, the programmers have to balance a team of 4-10 summoned nasties, along with making the summoner/necromancer reasonably interesting to play (nobody would play them if they summoned 10 monsters then did nothing, they need to be active if not nessiceraly powerful.
That's all for the programmers though; as a general rule I, as a player, do enjoy playing a summoner class. In Anarchy Online I always liked the Meta-Physisist (if that's spelt right it's a miracle) or the Engineer and in EVE Online I play with a very drone-based loadout (PvM, for reasons obvious to anyone else who's played EVE).