No, dude, they really do svck. If I try escorting them without assisting in any way they either end up all dead or most dead and run away. They don't even do their job. They scam the Lone Wanderer into doing their job and repay in chump-change. All they do is smoke, drink, sit around in their old, unventilated bunker and smell bad. They do nothing to be a significant force in the Wasteland.
Don't take game mechanics into account -- it goes against the point.
The Rangers do in fact complete missions; just ask them after you've finished the quest. You don't see it in game, but they still do it. Well, you do actually see Donovan, but that's a random encounter.
Their health is low, true, but that's Bethesda's fault. If you think about all they've been through, it'd make sense for it to be higher. And they're not really meant to be a 'force'. Their a private group who take on contracts for profit, not to have a huge effect on the wasteland.
Think about it: they survived on a roof with hardly any supplies and low morale whilst fending off super mutants. By the time the LW gets to them, they've been doing that for quite a while.