Buff specs B and C to 100 DPS (in theory). The sims will then show 100, 100, 100, which means that many players can change to B or C without looking dumb because they chose a DPS loss.
There are many potential reasons to switch to B or C: you may like their theme better, you may be tired of playing A for six months straight, or they have some utility that you feel could give more depth to your playstyle.
I think most people just want to feel they can play their favourite spec without making a "bad" decision.