If ARES missions are spawning TOO often, they'll spawn right near each other. At that point, it becomes trivial for players to shoot a couple badguys in each of several before dedicating to a final mission, and thus get rewards for all of them.
If you had three squads of 5, for instance, and ARES missions were spawning often enough that there were three in close range, it would be trivial to have everyone kill a couple badguys in two and then hit the third, moving clockwise, and have all three squads finish the third mission in their loop. This means your squads are now effectively clearing three missions in just over the amount of time it would take to clear one.
Heck, I already do this with thumpers and ARES missions as I drive past them; it's like free XP and resources. If they were more widespread and prevalent, it would be even easier.
Bear in mind, too, that the game is still early beta, and has only gotten a couple stages into the intended PvE content. There's a set of storyline quests coming, and quite possibly a bunch more active content. They can also scale up Chosen attacks with players, as well. I don't think Firefall's designed for players to log in and say "today, I'm just doing ARES missions". It's more chaotic and reactive. The world is actively changing, and the player experience is more in reacting to that world than in planning out a path. My usual play sessions will start out with me aiming for something, like thumping for a resource I need for battleframe progression or trying to get all the SIN imprints in a zone, and then I'll get dragged into taking down a tornado, and oh look, an ARES mission (either threat 1 that I can solo or higher but there's others heading in already), and oh crap Thump Dump is being invaded I'd better shut that down, and then it's like three hours after I logged in and where did the time go.