"I think it would help stop jetpackers that just sit up there, above reach by any alien"
Where? What place you mean? Tell me some.
Technically it doesn't matter if there is one yet or not. There *could* be one, since anyone can map for trem, but I see where you're coming from and agree with you on the rest.
Addressing the original issue of jetpack's use:
The aliens can similarly hide in some vents all game if they like, but like jetpackers, they won't be able to protect their base from there, usually. Sky-camping doesn't serve much purpose, but the jetpack is *not* without purpose.
Jets keep you well above rants in some maps with high ceilings, though still within view of your base. Jetpacks allow humans to get into vents to get those last aliens holding out for timelimit, and jetpacking above ATCS is an excellent way to catch the last dretch in a game, and can also allow you to snag a fleeing goon every so often.