Having been on both sides of this equation (using cell phones with or without cellular service, and having used standalone GPS units), I've found that ever since I purchased my Garmin 396 LMT-S and downloaded the Garmin Smartlink app, I don't even want to use my phone for navigation anymore.
It's quite nice to have a GPS that just works, doesn't care if I'm wearing gloves or not, can be easily read at high noon or darkest night, is easily removed and placed in the trunk, and doesn't give a whit if it's hot or cold, sunny or wet. Every smartphone solution I've tried came with a caveat. My Garmin 396 hasn't.
Every use case is different, I suppose, but for my two cents a proper weatherized motorcycle GPS is just easy. And when you ride like I do, easy is what you want.
(Tip of the hat to Ryan F9 for that last line.)