3. It renders most wedding traditions meaningless
Most wedding traditions become obsolete when we view the institution from the lens of secular culture, but a few of our cherished rituals that couples most look forward to when planning a wedding are particularly hollow and superfluous if you're already living together:
The honeymoon can still be a fun getaway for a newly married cohabitating couple, but it lacks the specialness that's there when it's the first time that a couple has spent extended amounts of time together under the same roof.
A father walking his daughter down the aisle has long been a sweet symbolic act of a woman going from her parents' house to the house of her own new family, but even its symbolism becomes strained when she's long been building a home with her new spouse.
Wedding registries were always a way that two people coming from their parents' homes could get a jumpstart on furnishing their new digs; if you're already set up in a fully functioning household, there's no need for those kinds of gifts.
And though I can't say I'd be sad to see this one go, there's no point in hosting bachelor/bachelorette parties when the engaged couple's last nights living on their own happened a long time beforehand.