I think that being able to re-build killed creatures is a good solution, but I don't think its the only good solution.
I think that one way to maybe balance the ability better is to make the summons start at higher levels with more shards. So if you capture a 3rd Air shard, you get a 3rd level creature instead of a level 1 creature.
Is this ability abuseable by razing a shard and rebuilding it? I assume 10 gold is pretty cheap for a creature.
Also, what happens when you capture a city that already has a shard built?