ATVs are most definitely very useful vehicles for a military but a lot of people have wrong perception of them, in part, due to video games.
ATVs are method of getting from point A to point B. Plain and simple.
They do have weapons but they are there either as cargo or as extra firepower in case of ambush.
ATVs are not combat vehicles. Their purpose is to quickly get near combat zone where you dismount and proceed into combat on foot.
They are meant to reduce legwork of infantrymen and provide space for some extra gear that may come in handy but which would otherwise be left at the base due to weight.
ATVs are not supposed to do the job of jeeps that you mentioned. Jeeps are proper vehicles, ATVs in military are simply another type of additional equipment. Do not let the fact that they have wheels fool you.
When you use standard unarmored vehicles for troop transport you are making a huge gamble. If shell fakes out off-roader or pick-up you risk losing up to 10 men or more, in case of tarp covered truck 20 or 30. If shell hits ATV you lose 2 men at most.
But like I said, ATVs are just additional gear not troop transports.
ATVs are cheap. Extremely cheap. You can buy trainload of ATVs for a price of used export version of average unarmed APC.
Because of this troops can treat them as disposable gear. If you are in a hurry to board the helo, for example, then just board the helo and forget about ATVs. Leave them behind and never think about them again because you have ten more in storage back at the base for every one you left behind.
Like I sad, cheap...
Maintenance is piece of cake. Mechanicaly wise they are glorified Vespas.
They carry more than enough cargo for their size and a lot, lot more than two men could ever carry on their own. You are more efficient in combat if you did not have to hike 10 miles to reach combat in the first place. Also, you got there lot sooner.
Your temate got shot and needs evac ASAP. Having couple of ATVs parked half a mile back could literaly be a lifesaver in a pinch. We are talking about difference between 5 minutes and 60 minutes before reaching medical help.
As long as you do not try to use them as you would in a video game, you are golden.
All in all, not bad for something that, unlike Jeep or something similar, is cheap, simple and, I can't emphasize this enough, completely disposable.
Back to bottling my Grenache