How useful is Bran as an Heir or potential Lord? The common conception (outside perhaps the extremely learned in Planetos) is that he’ll be unable to father children due to his disability, or lead from the front in battle. Was the assumption to the other Northerners that he’d always be a placeholder to Rickon if Robb died before producing an heir?

A placeholder can still be quite effective. Jonnel One-Eye, Barth Blacksword, Rodwell all never had kids, but seem to have been effective lords. 

Not leading in battle is harder to deal with, but I think Bran could get around it if it was known that he was a greenseer. The North follows the Old Gods and the greenseers are/were the closest thing the Old Ways had to a priesthood, so I could see Bran becoming a priest-king figure, venerated and feared. Good luck plotting treason against him when every tree and every bird is his spy. when the land itself and all living creatures on it are his weapons.