This mechanic is used in many game genres. Notably some are:
- Golems (a static building usually)
- Evolves (a moving unit usually)
- Lotrish games (normally static pre-placed buildings)
Other genres, like Diplomacy (Diplo Infinity and its clones), do not have unit spawning. Instead, they reduce the player focus on macro by having resource spawning (= control a building or location generates resources). The player then must still build production structures and manually construct units as would be in a normal game.
A problem common to all these uses of spawning is that each instance is fixed, pre-defined, and hardcoded. Players cannot create new spawning structures (unit factories, whatever), because the spawning location and requirements are hardcoded.
It is very feasible to allow dynamic spawning. That is, assume a Barracks creates 1 marine every minute via triggers. So, if a player controls 5 barracks, he should get 5 marines every minute. But, each marine should be spawned at each specific Barracks. The old way of doing this makes it impossible, unless the locations of the Barracks are fixed.
Instead, by using a for-loop through taking control of 1 barracks at a time to another player, applying the effect, and then taking control of each subsequent barracks before returning them all, a player can have unlimited unit spawners at any location.
The only maps I believe make use of such a powerful mechanic are Desert Strike and its clones.
I have not seen any other custom map that uses dynamic spawns. I have, however, put together my own map which uses dynamic spawning: http://www.staredit.net/topic/16412/
Why is this technique not used in many maps?
None.