I'll start by repeating that I think the Space Program's prime objective should be getting people permanently on Mars as soon as possible.

Preamble aside, setting up a manned space station at L-5 does make some sense. There are three reasons to go there:

1) To service New Generation Space Telescopes.
2) To investigate the effects of radiation unshielded by the ionosphere upon human physiology.
3) To gain experience with putting humans in deep space where if something goes wrong they don't have the option of immediately fleeing back to the Earth's surface.

Having said all of that. I suspect in the end, a space station at L-5 would become just another money eater / white elephant like the ISS. We should avoid distractions and proceed directly to Mars.

Gary