Shared Estates are good for a few things.

You may own a house, and not do gardening, but your friend may want to. I'm sure there's some RP tied to that specific situation as well.
You got a house in the spot you always wanted.. start to try and deco it and realize you aren't super good at decorating, you hire a decorator , or have a friend come in and decorate it.

A couple playing FFXIV together decide to share a house. One of the people who mentioned I try ffxiv out gave me full housing rights to his house for like the first 2 years I played. I've moved worlds, made other alts, and been all over, and no longer on his house but we're still great mates, and meet up and do dungeons and treasure maps together, almost on the weekly. I'd say that one experience helped make me a long term subscriber, and interested me in housing the most. Probably one of the 'warmest' social experiences I can recall from this game, outside of starting and leading my own FC.

One thing I've done to help people learn how to float is add them to my house and let them practice with a stage panel and wooden loft or riviera wall shelf, and out of maybe 7 or so that I did 5 actually learned how to do it and still deco houses and send me pictures of stuff they deco'd occassionally. I do think it's stiff tho, that a friend can't 'update' a house to keep it fresh, even if were just a 1 time deal. Kind of reminds me of old school ultima online, where a friend could open the door to your house and reset the decay timer in case you were on a deployment or couldn't log in for medical reasons. There was a big house a friend I knew got, and he met a woman from the game and they eloped. I kept going to his castle full of rare items and refreshing but I forgot to and he lost everything. I never heard from that guy tho, after he and she eloped. Maybe he finally found the treasure he was really looking for tho.