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?

the creature does respawn. i'm pretty sure it does

The creatures themselves aren't THAT useful - I use the air ones to scout and the earth ones aren't terrible.   They do artificially increase your faction power to possibly help you keep up with the AIs.

They do respawn - after 10 turns. And with each shard upgrade, the creatures get tougher.

The death shard demons are amazing.

I love Binding.

Which makes them far worse then normal troops and thus not worth putting in your main stack because stack sizes are so limited. Which in turn means they are only useful as back up troops or city garrisons, and thus aren't that good.