So having monsters usually drop only as many gems as they have hit points feels really weird. It means it seems like the optimum play is to avoid, so that you don't waste precious gems on killing things you don't have to.
Yeah, right monsters drop as many gems as they have hitpoints, with a 33% chance to drop more (1 health extra is 3, 2 health extra is 3, 3 health extra is 5). Crystal seed drops 2-4 minerals, so an average of 3. So using crystal seed, 3 plots = 3 crystals on average. Using only thorns on one-hit enemies this works out to ~the same mineral result but with much higher risk (3 plots for 3 thorns for 3 1-hit enemies = a 2 mineral gain on average).
I think the main idea behind this approach right now is that it puts much more value on non-default seed types. For a long time thorns were way too powerful, so, a lot of the economy right now is built to counterbalance that. Being able to use seed types that let you kill multiple enemies from one plot become extremely valuable in the economy. The seed types that enable you to do this are thwomp, star power, poison, maneater, avalanche, laser seed, shotgun, pistol, projectile thorns, radius teleport, bomb, homing shot, quad laser, swing mace, red hot rock, mine layer, boomerang (18/30 of the non-default seed types).
Maybe make a max number of mobs a spawner can spawn?
This might be a more elegant solution to this problem. Spawners are relatively new to the game and so we just defaulted to the thing that made it seem like it'd break the game's economy the least.