How rare is this legacy move set chancy

You should power it up and put it in gyms to annoy people. Chansey's strength is it's high defense and stamina so it takes ages to bring it down, the moves are pretty irrelevant.

Bad legacy movesets on unevolved pokemon are probably not going to be worth much. It is not worth doing anything with it right now. Placing Chansey in gyms is more annoying for your teamates than for your enemies, because your teammates would have the train against it multiple times but your enemies would probably only have to fight against it once. If it has good Atk and Def IVs, wait to do anything with it until after you evolve it.

Actually, I think Chanseys on psychic-type moves are better than the P/HB one.

One might say you have a, Legachansey

Probably not worth much since Chandra will soon evolve into Blissey. There may be that collector out there who wants to have every possible combo.

I would save the candies for now and hope for release to be able to evolve it to Blissey (and get a re-roll on moveset to boot!)

I'm not expecting trading to be implemented.

I'd hold off and use him to evolve into Blissey.

Nobody cares about Chansey moveset. IVs are what matters because, like everybody said, it's all about their high HP.