Granted, the Wii U could easily do it, and the 3DS could do it with the inclusion of the add-on, but do you think they'll actually include the functionality for quick "c"-stick smashes again?
Probably, at least on the WiiU. There isn't much reason to leave it out (on the WiiU) that wasn't also present when Brawl was released, and Brawl has it. There are reasons to keep it, though:
- It'll make some people upset if they don't.
- It'll leave one of the two sticks on a number of controller options (the main gamepad as well as the WiiU Pro controller, and possibly the Wii Classic Controller, etc) dead, which likely feels wasteful from a game design decision.
I know a few people who preferred them due to them being faster than the normal input despite the lack in power, so I can imagine there's a group that wants it to return...
I believe the majority (although definitely not all) of the higher-level/competitive smashers utilize this. Not only for smashes, but also for aerials, especially retreating fairs and advancing bairs. There is most definitely "a group". There are also plenty of "casual" smashers who also utilize it as well, although I don't have as strong a statistical grasp on that group.
There's enough to be vocally upset about it if they drop it.
I'd be quite surprised if the secondary-stick smashes don't make a return on the WiiU version.
With regards to the 3DS version, it is largely dependent on how popular the add-on stick is. If its enough of a market to consider, it'll probably function as quick smash/aerial stick. If its a tiny niche market that never got a lot of sales, it could be completely ignored. I wouldn't be surprised either way.