I've used both dedicated bike computers and an iphone mounted on my handlebars in a waterproof case, and the only upside I noticed to the dedicated device was that it didn't drain my phone's battery. The UI wasn't any better and both were fine in the rain.

You are right. I researched this now and the details that were important when I last bought a GPS are now no longer relevant.

Other than battery life, some of the reasons people prefer a dedicated unit are: better aerodynamics than a smartphone+case, support for ANT+ sensors, more rugged in the face of falls (even for a smartphone in case), more easily controlled by hands in a bicycle glove, and the ability to still have phone service even if the GPS battery is exhausted.