My parents have always been clear that living with them during education advancements is an option. Their purpose in those times was/would have been to ensure that my brother or I could focus entirely on the education, without having to work multiple jobs, etc, that could negatively impact either the speed or success of the educational program.
That being said, I have no idea how they would have felt if the educational opportunities had either been extremely long (some Ph.D and Post Doc plans are *extremely* long term, I know) or were unlikely to produce a job (at least of some sort).
My brother took them up on this offer while he got his MBA and (engineering) masters. They would not accept any actual money from him, but my brother did make himself available to help with errands, chores, etc, as much as possible. They rarely let him, though, as they were very adamant that he focus on his classes, get grade grades, and get out of the program in the fastest time. He bought all his own ‘things’ – groceries (they didn’t often eat meals together), etc, so he really didn’t increase their bills – he was just taking up space in rooms they never used any way.
Amusingly, after he graduated, he just…..didn’t leave. I just don’t think it occurred to him that he should. He had gone through a terrible break up with his first long term Girlfriend during grad school (they were LD during) and I think he just liked the companionship of being with my parents. Because it wasn’t a real financial impact, my parents didn’t care about the bills – but they did want him to LEAVE THE NEST AGAIN! :p So they started charging him rent @ 1/3 the mortgage payment. (He was now gainfully employed, and could easily afford it.)
It made no difference. His reaction was essentially ‘oh, of course I should pay rent now that I’m graduated and employed, sorry I didn’t offer to before!’, but he still didn’t leave! He did finally leave, months later – but only after he had a date with a girl who wanted to pick him up at his place so they could drive one car somewhere….
Anyway, the tl;dr is that my parents refuse financial participation if we’re in school. The argument is “I’m not taking on a boarder to make extra cash. I’m trying to support my children in their professional development. Charging you rent during school doesn’t foster that goal.”