Good timing. Moments ago, a mobile veterinarian just left my house after a sick visit with my cat. We are in DC, but I've had this set up in Seattle, too.
I would indeed have a mobile vet as a primary care Dr., analogous to a internist for a human. Said vet would have to either have on board OR have super quick access to a lab, though. Not having that piece would be a deal breaker.
My mobile DC vet is very willing to make a referral to specialists like opthamologists or oncologists, and that is reassuring. Let's be candid, those docs have $$$ on-site equipment that would be impossible for a mobile DVM.
For cats especially, a mobile vet (and a mobile groomer, FYI) is worth a lot. Good luck!