It just appears to me that Smash grew up to become a universal celebration of gaming in general. And therefore, it's able to overcome supposed boundaries to include as many iconic characters as possible. I mean, that's what they were clearly trying to do: Make people lose there minds with announcements that come right out of nowhere. And of all of the represented companies that have a special spot in gaming history (like SEGA and Capcom), Rare is one of them. Especially to Nintendo. I won't talk again about how well Banjo fits among all the Smash characters (im both design and moves), but he's certainly the Rareware character. It makes perfect sense to add him to the game.
To be fair, putting Banjo in Smash would be the highest boundary that would've been overcome so far, but this is at the same time exactly the level of insanity that they're shooting for.
But as you said, rocho, we shouldn't expect anything not be disappointed. Some of the reasons against it might've tipped the scales, so there's no telling at all. I'd be happy for almost every new character and Banjo would've just be the totally oversized, sparkling and eventually exploding cherry on top