Personally, I dont think you can have to many summons. The summoner has to have the stats done and keep track of buff. you forget it goes away or you dont get the benefit.
I had someone tell me in our last game session that we needed a mage with more offensive spells. I had to explain that my one creature MSIII did more damage than the fireball I could have cast. the creature also allows for flanking and possibly other special attack.
The problem i run into is that many of the bad guys have protection from good up and so hedges out my creatures. Of course two huge monsterous centipedes can block a lot of space and keep the bad guy boxed in.