You can make do with what you have (higher mags are not a "must"), but if you are an experienced scope user and find it satisfying to make IDs of distant critters even though they lack the immediacy of those more close at hand, you will find much use for higher powers, especially in Hayden Valley, in meadows below Dunraven Pass, and in the Lamar Valley. Watching wolves and grizzlies on elk kills, wolves and their pups around dens, Harlequin Ducks in LeHardy's Rapids, moose browsing on willows north of Norris, and other sorts of behaviors can be especially satisfying at high mags. For strictly making IDs (rather than behavior observation), 30x is adequate. For that matter, 8x bins can be used for most ID.
--AP
PS - What time of year are you going? During the warm seasons, if you are at one of the larger turnouts, you'll find at least a few folks who spend the whole day there happily letting others take looks through their big Swarovski scopes at distant wolves or bears doing awesome things. All of the above said, as someone who has made numerous trips to Yellowstone, my best advice is to get up early. In June, I like to leave camp by 4:30 AM, which gives me about 5 hours before most other park visitors are active. At that time of day, you will find numerous e.g. bears right alongside the road and very nearby along river access points. No need for a scope when grizzlies are mating 20 feet in front of you!