The Seer and Devil aren't supposed to be equal. The Seer is fundamentally more important regardless of how much detail you give to the Devil; really, the Seer is the most important role in Classic. Meanwhile, even if you give them equal power, they're not equal. Say for example that you remove the Fool result, so each receives only Villager or Wolf as results. The Seer is then just as powerful as before, and the Devil is a weaksauce hinderance to team Wolf, actually worse than just a vanilla wolf. Occasionally giving them a "this guy might be the guy we want to kill" only helps the Devil somewhat, and once the Seer inevitably outs themselves, they're once again worthless.

What can happen if there is a weak Devil is an open network--indeed, people have found that optimal strategy with no devil, alpha, or fool is for the Seer to claim D1. The baner protects them every night, the wolves have no way of finding the Baner except killing at random and hoping, and the Town may be best off not actually lynching anyone unless the Seer finds them guilty--just to ensure that the wolves have no help finding the Baner. Then on the day after the Baner dies the Seer announces their innocent scries and Town gets cracking with 1/3-1/2 of the wolves dead and twice to three times as many Masons as there should be. The Fool will prevent Town from starting such an extreme strategy on Day 1, but a weak Devil makes find-the-baner races so much more painful to the wolves than they should be. Indeed, in such scenarios, the Devil is useless as they cannot tell the wolves anything they don't already know.

I know that these are the rules as he understood them in 2006, that he's brought WW here though he's hardly the inventor of it. Yes, the rules work. My point is that this rule isn't like the PM rule that changes the nature of the game, but simply an improvement: the game is more stable and less likely to devolve into Seer tedium with a Devil that's actually useful.