I think it's precisely because personal housing is mainly a luxury that it should be the intended use for these housing wards.

From the looks of it, it doesn't look like the limited availability of plots is going to go away soon - so just give it to private owners, and if you miss out on it, well as you said it's just 'glamour' so it's not a big deal.

FCs should be the one using instances to access their headquarters (or whatever you want to call it), because those can be generated freely and a homeless FC loses a lot of functionality. Even if you banned plots from being used as personal houses, you would still have the problem of FCs with no house because there are way more than 2160 FCs on some servers.