If you have multiple houses, your stashed gear is all separated out and you dont know what you have
If you have multiple houses you have to spend almost 100k gold for what?
There is no real benefit to having them all in game
What else are you going to spend gold on when you've decked yourself out in the most mystical of artifacts discovered in deep dungeons?
Plus, if you don't fast travel, you don't have to walk as far to store your stuff.
Also, some of the houses are a bit small, like Breezehome. Sometimes you just need more display space.