I think Atlas has somewhat spoiled us because of the availability and low cost of their parts. It certainly makes thinking about a kitbash of one of their locomotives a lot easier than most others. Kato runs a distant second, yes they have parts but at the usual cost of $25 and up for a shell (unless you catch a sale) it's not always a no-brainer. Bachmann also has a decent parts department, but their source of parts through warranty returns makes availability spotty.
I know keeping a parts warehouse is expensive. If business allows it, that's great. I do not, however, view it as a moral imperative - it's a perk. I would hope that a manufacture could at least afford the level of service that Pud outlines though. I would consider that the mark of a good company that cares about it's customers. Down the line, it may or may not make a difference in a purchase. Most likely depending on how badly the item was desired.