I followed up with Dick with this, but thought it might be of wider interest and probably generate a few comments, so I'm posting here as well:
A few more things I wanted to point out about using a phone-based GPS app on the bike...and that's operation, power, heat, rain, and custom routes; this is based on my experience using my iPhone and the TomTom app on my LT as my only nav device for a couple of years.
If you don't have the device plugged in the whole time, it will suck your battery down really quickly. So you have to have it plugged in the whole time. In fact, on my iPhone 3S, the TomTom app would draw power faster than the phone would charge; so the battery would run down even while plugged in. If I turned off the screen, it would be OK. So I'd have to manually turn it off when I'd get on a stretch where I didn't have any turns to worry about. Then I would have to remember to turn it on and check every once in a while, or if I wanted an ETA update, etc. Not insurmountable, but a pain in the butt.
And working the phone with gloves, even to just turn it back on, is hard. The iPhone screen is not pressure-sensitive, but capacitive-sensitive, so it will not work through gloves. You either have to get gloves with a little patch of conductive thread (or sew some in yourself) or cut a little hold in the thumb or finger of your glove so your finger can touch the screen; the latter is what I did. But even with that, working the phone with the bike moving is tough; the screen is so small that anything is a challenge; and of course it can be a safety issue if you're not really careful, as it's easy to get distracted if you're struggling to operate the phone.
If the phone is in the sun, it will overheat unless it's getting some air, and maybe even then if it's running a nav app with the screen on the whole time. Once I moved mine up on the clutch reservoir with a RAM mount, along with turning the display off most of the time, it would do OK. But if you try to, say, put it in the map window of a tank bag, it will overheat really quickly and shut itself down.
And then of course there's rain; you have to be ready to put it into a waterproof place if you get into rain. That works OK if you're getting voice navigation, but obviously you can't see it, and you probably won't have power, so your battery will go down fast. Or you could use one of the waterproof cases to mount it, but now you're potentially back to the power and heat problems.
And none of the iPhone apps that I'm aware of let you put in custom routes. So you have to keep your waypoints in your head, and then route from point to point manually. This can be very tedious and error-prone if you're trying to follow a scenic route, or hit all of the twisties.
All of this makes me glad I have my Garmin on the new bike. It's always plugged in, I can always see it, and it will run all day in the sun or in the rain. I can plan my routes on the computer and then download them and be confident that I'm going to get what I want.
Of course I paid about $600 for those advantages.
It's a classic 80-20 situation. The phone apps do most of the job for a (very small) fraction of the price of something like a Garmin Zumo 6xx. But there's a reason Garmin can charge more for special-purpose devices. They have a bunch of products aimed at boating and hiking, too. I think this is what Garmin is going to have to survive with, as they will clearly lose a lot of the general auto-based GPS market to phones and factory-installed units.