Considering that if you're still a stage 1 alien (whether your team has s2 or not) and you're against stage 2 humans, you want to either get stage 3 or massacre the human base really quickly. In other words, you want to get your kills and not die. Going against a s1 human is more sure than against a s2 human.
Basically, unless the humans are massively out-skilled by a dragoon, the dragoon's kill rate once humans hit stage 2 drops severely. You can get stage 2 humans as a dragoon, and the better you are the easier, but those 2 to 4 hits added to the dragoon's slow attack rate make any larmour+helmet a threat. I'll almost never charge into a group of more than 3 stage 2 humans as a dragoon because 200 hit points don't last forever.
That said, if you find yourself in a 1v1, you should try to prevent the human from hitting you. If you can get an initial pounce, that saves a lot of time. Try to stay behind the human but if you find yourself being outmaneuvered, you may have to try faking the human out and retreating.
If it's 2 goons against 1 human, back off. Either that human will die in a fraction of a second or the goons will get in each other's ways.
--- Oh, forgot to answer the question. lol ---
How useful a dragoon is to me once the humans are at stage 2 depends on how good the humans are or how bad the lag is. Often times I'll just dretch until we get to stage 3 (and by then I'll probably have plenty of evolution points to get an advanced dragoon or tyrant).