Well you dont necessarilly have to create units like that,you can also have them waiting somewhere (out of sight) untill you need them
For example:
Airstrike:
Place a pilot near the runway
Place en EMPTY A10 on the runway
Give the pilots a MOVE waypoint close to the A10
Then followed by a GETIN waypoint on the A10 and the waypoints for him to go to for the airstrike or airsupport, and perhaps some MOVE WP's in between or afterwards and landing at the end, whatever suits you!
Now place a trigger, radius not important can be 0, type switch, activated by radio alpha (or whatever channel you like)
Synchronize this trigger (F5) with the first MOVE waypoint (do this by holding down left click and drawing a line between trigger and WP)
now the pilot will stop at the move waypoint and wait for radio call 0-0-1 (alpha)
So when you call him he will continue with his WP's thus getting in,taking off and flying his airsupport mission.
You can do the same for other units or groups aswell, and also just interrupt any units/groups path between waypoints by using a trigger on one of them, these triggers can be activated by radio but also by other events like enemy units reaching a certain point, or friendlies taking casualties etc.
For example, tanks needed when enemy tanks enter a town you hold:
put the tanks some where on the map, which is their starting point, out of sight of player,
Give them waypoints for all their actions and put a trigger into the town, which is type switch, activated by east.
radius here has to cover the area the enemy drives into.
now just synch this trigger with the tanks first waypoint (which is real close to their starting point). and they will start to roll when the enemy arrives.
So you see you can have units and groups "appear" like that, they dont have to fly/drive or run around endlessly untill they are needed!
Hope this helps
MadFred