Not "necessary" but it makes things a lot easier once you get it set up. Change settings once in OwnerRez, they sync out to the channels automatically, bookings come into OwnerRez automatically with guest info, calendars update instantly, etc.
In terms of drawbacks, it's more complex because you have now 3 systems working together (OwnerRez, Vrbo, Airbnb) instead of separate systems.
For Vrbo, OwnerRez will process the guest credit cards on your card processor (so you must have one), and taxes will come to you so you have to remit them -- so that may be either a drawback or an advantage, depending on your case.
