I think that's not a bad idea to have a fuel-range ring or some other kind of indicator. The software could adjust as the flight goes along, based on your aircraft's profile (fuel capacity, climb/cruise/descent fuel burn, and climb/cruise/descent speeds). However, there'd have to be some way for the user to be prompted to provide the current fuel load before each takeoff. It doesn't now. So no, you're not missing something.
I'd rather see an engine-off-glide ring first, as that's a more of a critical thing in an emergency, especially if you're flying through the massive mountains we have in the West.
To get a similar result now (manually), you can press on the screen (or click and hold with a mouse) and then drag. The software will show an ever-growing cyan colored ring around your aircraft along with a the radius of that ring in miles (nautical miles I suppose, if you have those units selected). If you can't drag it far enough to get the full range of miles for your fuel load, then first shrink the scale down on the map, then do the press & drag.
I don't remember what the current production release of the software shows in Flight Plan. I thought it showed a running total of time, distance, and fuel for each leg of a flight plan. I guess not. The current beta version does. Also, in the beta version, you can indicate that a landing will be a fuel stop, in which case the software will do a running total of time/dist/fuel from takeoff to that stop, and then start over until the next fuel stop or the end of the plan. Currently, it doesn't give you a grand total for all three at the bottom, but I imagine Brian is going to add that in so we won't have to fuss with adding things up manually if there's a fuel stop(s) in the middle of the plan. If there's not any fuel stops, then the last entry in the plan shows your totals for all three.