The problem with a horde scenario vs a fireball is that with a fireball you pick a focal point and roll. With a horde you must move each minion to its proper space, taking into account a lot more tactical decisions including flanking, OAs, Cover, who attacks when syncronizing with diffirent powers and abilities, beccause who says you can only have a horde made up of one type of creature, etc.
The problem is that you're still thinking in 3E terms of what summoning a horde means. If you summon a tactically complex creature, there would just be one. If you summon a horde of creatures, they would not be tactically complex. It's as simple as it sounds.
To be more specific, here's some possible types of summons:
1) Full Fledged - Summons a tactically complex, comparitively powerful creature, which takes at least your standard action to control. So you're basically replacing your actions with the creature's.
2) Lesser Creature - Summons a relatively weak creature who's options are always just "charge someone" (or something equally simple), which takes a minor action to control. So you're making one decision for one of your actions.
3) Horde - Summons a horde of creatures who attack everyone within their area. Like a swarm, you don't keep track of individual creatures - everyone in the area gets attacked, like a zone, and the entire thing has one pool of HP. The creatures wouldn't get OAs, so moving through it would have a static effect like difficult terrain and/or taking damage. Standard action to make the whole horde attack. So the only decision you're making is where to place the zone as a whole - not every individual creature in it.
4) Single-Purpose - Summons a creature which locks onto one foe and keeps attacking them as long as it lasts, not under your direct control. Or it simply stays in one spot and attacks people who try to pass it. No action to control, because you aren't. So you make the decision once when you cast it, like a spell that does ongoing damage.