In several of the missions/instances that I've done recently (Toy Soldiers and the Knead Caketastrophe one), a room is blessed with two or more auto-spawn units. The first time you kill them they reward you with a drop, but after that they are fairly useless for rewards and just seem to server as motivators to keep you shielding and moving in the room. In the case of the Caketastrophe one, ignoring them and moving to the next room seems to be the solution, but in the Toy Soldiers missions they accompany a nasty Battlepod that can only be attacked between cycles. I've tried to time the killing of the Mechs so they do not spawn at the same time i am attacking the Battlepod (next to their spawn point) but it is a bit challenging to say the least.
Is there a 'better' strategy for dealing with aggro from enemies that never stop coming, yet provide no reward?
You are right that the respawning enemies don't drop loot. They do sometimes drop hearts though.
The last fight of the second level of Ironclaw Munitions Factory is a good example. There are three spawning points. Put one party member on each one, with his shield activated, and he will "block" the spawn.
If I recall correctly, this tactic does not work in Toy Soldiers. Camping on the spawn point just causes the mecha knight to appear next to you. I'm not sure why. So you have to defend against the mecha knight while you wait for your chance to kill the battlepod.
Here's another idea. When the monster spawns, don't kill it right away. Just dodge it for a while. Then kill it just before the battlepod lets its shields down. Then, at least you can attack the battlepod in peace. I haven't tried this tactic in Toy Soldiers, but it works in another mission with gremlins and battlepods --- Sewer Stash maybe?