We visited Yellowstone 2 summers ago and stayed in Fishing Bridge for a full week. While Fishing Bridge is an RV parking lot type setting, we used it solely for parking the RV, eating dinners, showering and sleeping.
We were gone from the site from 6am until we were done sight seeing every day. Fishing Bridge was a great location for us and we would definitely stay there again if we go back. We wanted full hookups. Did not want to move and set up camp again. Did not want to worry about moving camper without reservations. Fishing Bridge allowed us to get out and experience the park without having to make the 30-60 minute rides in daily through the various entrances.
Yes, some parts of the park were far away from from our location but with careful planning and early starts, even the farthest reaches of the park were a pleasure to travel to, after all, you're in Yellowstone!