I think this is working as intended, however I agree with you, that if the mana cost is the same, and it goes away after battle, you should get a higher level unit for use in the battle. Either make the behaviour of the spell differant if casted in tactical, or if that is too difficult, make a seperate spell for tactical summoning.
Edit: On another thought, perhaps increase the level/strength of the summoned creature based on number of shards as well. Overall, summoned creatures are pretty weak when compared with regular units that are trained, especially since you can give bonuses to regular units based on buildings. Should have some similar effect for summoned units, either based on intelligence of caster, or number of shards.