Boron, do you mean that if given the choice of having 50 Winter Wolves or 40 Winter Wolves and 40 Coral Guards you choose the 50 w-wolves? What if the choices are 100 Lamias / 70 Lamias, 20 Knights and 30 Longbowmen set to Fire Archers?
If you can only choose between all-summoned and all-mundane armies you should choose the summons to be competitive in lategame. But DomII isn't HoMM4*, and you don't have to. You can use both mundane and magical units in a nice, joyful and often colourful mix that is stronger and more varied than either of the one-sided armies. You might have a mage or two less, but you don't have the gems to use all of them for summoning if you only recruit mages and build fortresses/labs to make more of them, not recruiting any national units. And besides Mictlan every nation has something useful. If nothing else, archers that can be Wind Guided and/or given Flaming Arrows.
Endoperez
* I never played it, but I have heard that it forced you to choose between good in beginning/bad in the end, bad in the beginning/good in the end and mediocre at all stages. And AI had access to the best units from beginning to the end. If this is not the case,

.