Spikes, Tspikes, and Sleep Powder are big perks that Roserade has and Celebi doesn't. It also can be used as a Tspike counter, where as Celebi hates Tspikes. Whilst Celebi does get SR, it shouldn't be hard to fit rocks onto another member of your team given how widespread it is. Spikes, Tspikes and sleep are harder to do so. I wouldn't conclude that Roserade is "completely eclipsed by a Pokemon in the above ranks." Rather, the B rank definition is more fitting: "Pokemon who are partially outclassed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category."