There are actually quite a lot of reasons to want it, even if you were under the context-less maxim of "third parties shouldn't be tied down to a particular platform."
Being under Nintendo's umbrella would provide stability, steady work, and an infinitesimally small chance of being closed for underperforming. I can't even think of a studio Nintendo owns that they outright closed. The closest one that comes to mind is when they re-purposed Brownie Brown into 1-up Studio. Nintendo occasionally allows their subsidiaries to do what they want, or at least how they want, like Monolith and Xenoblade, or Retro and Metroid Prime. The thing that led to Scalebound being cancelled was Microsoft's mid-development meddling to get the studio to follow trends in online multiplayer. And there's also the benefit of surviving this rocky industry. Platinum already struggled once after Sega refused to publish Bayonetta 2 and Nintendo came to the rescue. As former Platinum producer JP Kellams once put it, there are a few people who would rather Bayonetta 2 not exist at all than exist on a platform they don't like. Some would damn Platinum to smash against the rocky shores of a tough and unsure industry than find safe harbor in an unwanted port.
I'm not sure Nintendo would go for it, though. It depends on their valuation of Platinum as a whole and what they can bring to their business, especially in the longterm.