If you put too many requirements on developers they may just decide to not support you at all.
And yet developers still eagerly support Microsoft (despite Achievements being mandatory this entire generation) and Sony (whose trophies became mandatory in 2009). There's Steam Achievements on PC, too.
Brood, I don't think it only being optional is going to have much of an impact on how many games on the Wii U support it, and I will tell you exactly why: because in the case of multiplatform games the work for the achievements is already done. They can reuse the art from the other platforms and just copy and paste it into their Wii U port as well. So it would be a trivial thing for them to implement. Its not like they would have to make new artwork or whatever for each console. They just do it once, and then reuse it over all of them. Or they could just cheap out and use some generic artwork for every single achievement. I've seen a lot of games on the PS3 where every trophy shares the same image, which sucks, but at least its better than nothing I guess.
The only time where this might be a problem is in exclusive Wii U games which aren't multiplatform, so in those cases they can't copy and paste them from the PS360 version. But we all know 3rd party exclusives on Nintendo hardware are few and far between.
So the bottom line is even though in theory the feature is only optional, I think in practice it will be used in most cases. You also should take into account the pressure fans might put on developers to include them.