Just to clarify a bit, Tartare (I guess you're asking in reference to the Kiwi Huey thread on the Military forum):
Lots of helicopters, including the Huey, don't have an airspeed indicating system that will read below about 40 kts. You do have attitude indicators, but the hover attitude is probably about 5 deg nose up and a few degrees left skid low, and will vary depending on loading anyway.
The bottom line is that if you tried to hover (or climb in a hover attitude, ie trying to stay over one spot) with no external reference, you would in reality be drifting around all over the place and it would be extremely hard work.
Secondly, and more importantly in this case I think, a heavily laden Huey may well not be able to hover out of ground effect (OGE), so if you tried to slow to the hover in order to attempt a vertical climb, you might well run out of power and end up going down anyway.
It would be great if you could do it, but having practiced trying to hover on instruments under a hood, it's damn near impossible because you don't have any idea of which way you're drifting - you could be doing 40 kt forwards (or backwards or sideways) without knowing much about it.
For those guys, being in formation would have greatly complicated the issue too, as a formation is a lot less manoueverable than a single ship.