Hi Liz,
I signed up for it first thing when I started sitting through Rover. Even if you never have to use it, it helps in your search ranking, and at $50/year it's less expensive than even one potentially preventable emergency vet visit. I'm big on doing my homework on health issues with my own dogs, but when dealing with someone else's pet I'd want a professional opinion for a variety of reasons. First, it covers you as a business owner. Second, I know the baseline behavior for my own dogs, and I know how they act when they are hurt or ill; I don't have that kind of history with the dogs I watch, and they may act differently with me than they do with their owners. Third, in the event of an illness or injury, you can reassure the owners when you contact them because you've already spoken with a vet and you can relay what a professional has said to help them make an informed decision regarding whether or not they want their pet seen immediately, in the morning, or even if they need to cut their vacation short.
Beyond the vet access, you also get a year subscription to 1-800-HELP-4-PETS. They send you tags that you attach to your guest's collar when they begin their stay. Each tag has an ID number that is associated with your account. If they get free during their visit, anyone calling the number on the tag will allow 1-800-HELP-4-PETS to contact Rover and you, and reunite lost pets with their sitters faster. This way you don't have to worry about whether dogs you watch are microchipped or have current tags, and they can contact you directly rather than going through the out of town owners.
Thankfully I haven't had to use any of the features yet, but I'm sure it has helped me in the search rankings, improving my business prospects, so if you get just a couple extra nights booked because of the perks, it pays for itself.
http://www.rover.com/protection-package/