You join a legacy for the career prospects, fleet, command, seniority agreements, pension, union recognition, CV, staff travel etc.
That certainly used to be the case, it was pretty much a "given", but TBH I'd now second the health warning given by PorridgeStirrer..
At some European "legacy" carriers the sort of items listed above are under attack as never before, and IMHO there is increasing validity in the POV that it's better to take the money and potentially rapid advancement with a LCC now than banking on joining a "legacy" at the bottom of a large seniority list simply because jam is being promised tomorrow...