This was the way it was in WOM, and it was easily exploited. Well, not exploited really, as it was part of the game, but it removed the need for trained units, and allowed players to steamroll the map with early summons. When summons were reigned in a bit, players then moved onto steamrolling the map with Champions. We are just now getting to the point where we might actually want to build armies. Putting back in an easily replenishable source of early game fodder that doesn't require the use of any of the game's production mechanisms, would again allow champions to level very easily in the early game and be moving backwards design wise.
Summons are a very powerful ability, and very efficient mana-wise. Even weak ones serve as both offense, and damage mitigation by way of absorbing a couple hits, and probably for cheaper than the same cost of using a couple low level offensive spells for about the same damage. An early trained army unit would do the same thing, but that unit costs money, resources, and takes time to produce. Unless you want to make the mana upkeep on summons so prohibitive you could only use them situationally and assign a cooldown timer similar to a production queue, even for weak units it's a no-brainier to use summons instead.
It's good that they are buried later in the spellbooks, in my opinion.
