I am not sure that OP makes a good argument. Actually, he makes one argument - summoners are up. The second argument, it is boring to have just one type of summon per summoner is not really an argument, but rather a preference.
Regarding summoner being underpower - first of all, it is hard to speak about anything being underpower in a single player game. All default races are winnable - i beat game on ridiculous iron man mode with each of them, so it is doable. Some are harder than others - I think it is good. I am not really sure, while summoner should be as powerful as mage, or why all the races and heroes have to be exactly balanced in difficulty. I actually like it that with some races you have to think harder to win, and to use their advantages well. Saying that it is more logical to have more summons than just one, or that in other games they have it, is not really an argument either - it is not logical at all, nor does this game need to have all options other games have.
Regarding the preferences - "it is boring to play summoner with one summon" and "undead with 1 lvl cities" - well, those are subjective. I for change, enjoy this way more. I have my own personal preferences - I would love to have bigger armies (like 20 troops/army). Those personal preferences can be meet if their is unlimited budget team behind game (so they can include hundred of options - e.g. 1/2/5/unlimited summons per summoner), or by a game being modable. And this game has mods, maybe you can find something that fits your preferences better.