I would also throw my hat in the ring with those that feel summoner's should be able to summon more than one creature at a time. I created my character as a summoner/Necro with the intention of having an incarnate and bone spider out while playing with the bloated corpse when the timing is right.

While I agree that the incarnate seems quite strong, I think the fun factor of a build is just as important as raw strength, obviously opinions will differ tremendously on this. I find it to be a bit tedious and uninteresting to summon my incarnate and then proceed to buff him after he has been summoned. Obviously you can't add all of his abilities to him and leave him as a 2 AP cost summon, but I honestly would find it more interesting to have the player have to make a choice on a couple abilities to give to their incarnate. I think this would do two things, offer players a decision instead of just having the incarnate be a catch all of abilities and jack of all trades, as well as nerfing him a bit. With this nerf I think that it should open up the ability for the summoner to have multiple summon's up at once since the incarnate wouldn't be such a monster.

I don't find the totems to be a good substitute for having multiple summons as they not only feel weak but have very little player interaction. I make one choice with a totem and then forget about it, as it will either die instantly to an enemy or friendly AOE or it randomly chooses someone to attack regardless of what I am doing.

So in summary I think the incarnate being as strong as it is limits the summoner class to being less fun and interesting as it could be. The ability for the incarnate to be buffed to do almost anything limits the class to just summoning the incarnate and then buffing it to do everything you could ever need it to do. By limiting the incarnate to less skills and having the player make a choice as to what skills their incarnate has it would bring the power of the incarnate down and allow for more power to be added else where in the class such as summoning other creatures which I believe would make the class more enjoyable to play.