The only reason I can think of as to why Nintendo hasn't said what the new amiibo in game will do is because they are tacking on the functionality last minute.

Considering the development time and Nintendo's approach to amiibo, I don't think that is the case. I think the reason we haven't heard much is because they don't want to do much/anything special, so they're not making a big deal out of it.

Wolf Link will be pretty cool; the other new ones will be very superficial in their in game use.

Like, Wolf Link was "cool" because they basically just wanted to sell Twilight Princess HD at $60 and to detour the second hand market. By putting the amiibo in there and giving it a cool feature, that makes the complete set more costly for second hand, thus encouraging new purchases.

I like amiibo, but really only buy ones that offer some decent level of functionality.

I don't blame you, but I don't think amiibo will ever have "decent level of functionality," at least on a grand scale for a couple reasons.

• At its core, many people want amiibo because they're cheap, okay quality and feature more unique characters/titles than World of Nintendo. Even the presentation of the amiibo is better than a lot of what I've seen from World of Nintendo. They just start losing out to things like Nendoroids, which isn't fair to compare and will never include characters like Captain Falcon.
• Unlike similar games, amiibo don't have a specific game/use and are more general use across games that support them. Even if you don't like what Link does, it still does like 10 things, even if none of them are great.
• Nintendo seems to understand that the collector/display intent comes first, which is why they're okay with putting more effort into showcasing them, than actually implementing or talking about what they do.