Not only is Prince guest-starring on the post-Super Bowl episode of New Girl, it's come out that His Purple Majesty approached the show, not vice versa. According to an article on LAist, Prince is a fan of the series and specifically asked for a role where he gets the relationship of Nick (Jake Johnson) and Jess (Zooey Deschanel) back on track. So no, you're not the only one who's dissatisfied with how that story line has been playing out this season.
But seriously, we all know why Prince wants to be on New Girl: Prince wants to shag Hannah Simone. It's not like he's alone on that point or anything, but think about it: everything about Hannah Simone, from her gorgeous and ethnically-ambiguous looks to her name, sounds like she stepped directly out of a song on Around the World In A Day or Sign O' The Times. In a perfect world, she would have been on stage in between Wendy and Lisa in the Revolution. (I'm thinking her instrument would be trumpet, like Cynthia Robinson in Sly and the Family Stone.) She seems pretty much made to wear high heels and lace. Hannah Simone is so much Prince's dream woman that it's like she sprang fully formed from his brain like Athena. So it's kind of sweet, in a weirdly juinor-high-crush sort of way, that he'd devise this plan to guest star on her show just to hang out with her.