LOL. TBH, the only specific use for her is in MK8. All amiibo are compatible in Hyrule Warriors, Mario Party 10, Captain Toad, and Amiibo Tap.
In Mario Party 10, Rosalina can instantly unlock her own amiibo board and can be used as a character in amiibo Party as well. Even if you don't have her, you can collect her board piece while playing amiibo Party, but it'll take a while before you can play it. Any amiibo other than her, Mario, Luigi, Peach, Yoshi, Bowser, Wario and Donkey Kong will just get you a scratch ticket per day.
The least useful rare amiibo for me is Little Mac, Captain Falcon, Pit, Lucario, Jigglypuff, Greninja and Ness. Outside of Super Smash Bros., they are the least compatible out of the bunch and are stupidly expensive to obtain. Therefore I would see why collectors wouldn't even bother opening their box.
Even though Pikachu is one of the most common amiibo and is likely to get games that support him, so far there isn't a single game that would unlock an exclusive feature if used. I know he's extremely popular, but why there were so many of him produced while he's among the least useful amiibo is scratching my head even further. The same can be said to Diddy Kong to a lesser extent.
. I open every amiibo I get, from Mario to Rosalina, use them for Smash and put them back on my nice display when I'm done.
