Hello, massive wall of text here.
I played WoW from Vanilla to WoD, and a bit of SL. Also been playing FFXIV for six years. FFXIV has housing. It's far from a perfect system but I'm not here to complain about that, I'm just going to list how it can be a good thing for a mmo.
Housing can be amazing for social reasons. Having a place your guild owns can be great for connecting to the other members, particularly if you're new. It can be daunting to be the new person in a group, so having a location where only your fellow guild members go can make it much easier to get to know people. Sure you can bump into players in a city, but with a house players don't get lost in a crowd like they can in a major hub. Also some people find it easier to start chatting in more intimate settings.
Players can also have their own private chambers and many find it fun to see how others decorate it - it's a great conversation starter. My guild has had some contests for the best decorated private chamber.
My guild's house gets redecorated to match in-game events (xmas, easter, etc). The members always look forward to this and take a lot of screenshots together with the festive furnishings, often with outfits to match.
Needless to say housing is a fantastic tool for rpers. Not only do rpers get a place to rp in peace without trolls interfering, they can also create any sort of location they like so they are not limited only by what locations the devs have created.
Houses can have many convenient things in one place that you use frequently. Repairs, guild bank, personal bank, etc. You don't need to run around to access these as much as you would in a city because they're all in a much smaller location, and placed as close together as you wish. You can also have a target dummy in the garden to practise your rotations.
Housing makes crafting and gathering constantly relevant. Cosmetic items don't lose value at the same rate as items with combat power attached to them. Especially if creating said cosmetic item is a potentially lengthy process. Most furniture in FFXIV is crafted so you can craft to decorate your home, or make a lot of money selling your furniture.
As for more personal reasons to have a house, it's very relaxing content that doesn't really have a definitive end. Your house is finished when you decide it is. It's a nice change of pace to play the game without any sort of expectations making you play in a certain way. You just do whatever you want. It's also a great creative outlet. With a diverse range of furniture you can make any sort of house you want; a fishing shack, a restaurant, a church, a night club, a noble's home, a haunted house...the possibilities are endless. Some players in FFXIV have made their house into one giant jump puzzle.
And on the topic of creative outlets in FFXIV there are actual interior decorators who offer their services in exchange for in-game money. Decorating in FFXIV can be very in-depth, especially with the use of glitches. Some players are just wizards with it. (as an aside the FFXIV devs don't mind that players use glitches while decorating as they view it as harmless fun).
As for how housing could be in WoW, most of the above would be applicable if the system ends up being at least as flexible as FFXIV's housing. WoW has a huge amount of competitive players so housing would be a great way for players to show off trophies from their various exploits. For example a guild who kills the final boss in a tier while it's still current could get a trophy only obtainable in that window. A huge list of achievements could have their rewards adjusted to include furniture, which would make many players try to complete achievements they may have otherwise ignored. WoW's crafting is in dire need of something to make it much more relevant, and housing would be a great way to do it.
Aesthetically WoW has a lot of character. The huge variety of npc cultures shown through how their native homes are decorated would be so much fun for players to play with. Blizzard wouldn't even need to create much furniture to begin with...because it's already there! The amount of in-game assets is astonishing. A huge amount of work for housing is already done.
And while garrisons were very badly received by the playerbase, it does from a dev point of view serve as a baseline for player housing. And by that I mean some of the hurdles in working out how to make it instanced so that every player can have one has already been done. So it wouldn't be introducing an entirely new feature, but a reiteration with a different purpose and more options. In short - we know Blizzard could do it.
And anyone who thinks housing is just like garrisons...no absolutely not. Garrisons have very little customisation. At a glance everyone's looks the same. There is near zero creative potential with from from a player point of view. They also cannot be home to multiple players, so they're a poor platform for a social experience.