A number like [ Tank Aggro] - [DPS Aggro] would be nice to get a number on the gap. A little more annoying to calculate into DPS (since they don't like that), gives you an idea of value instead of proportion. In a fight where you might be out of tank stance for a significant part of the fight, it might be hard to judge exactly how much of a lead you have over the top DPS. A 100k gap in enmity is perfectly fine to continue out of stance, but if you see the gap is, say, 40000 you might want to consider using more power slashes and such.

That said, I don't think they'll add those numbers, so people who want to fine-tune their stance uptime or get a better grasp on enmity in the raid are going to have to either deal with it or use illicit programs.