I use gpsfiledepot.com maps on my 705. But I ride a mountain bike in the woods. I have no need for routing information.

Hm, why do companies like Garmin still exist?

1: there will ALWAYS be a market for purpose-built devices. not everyone wants to strap a phone to their handlebars so it can shatter when you crash and you can lose not only your GPS, but your cell phone to call for a ride (because your bike is now broken, too), your camera (to potentially take insurance pictures of the idiot who ran you over), your contacts list (so now you have to post on Facebook and Twitter for your friends to give you their numbers again).

2: cell phone companies STILL don't have a quick and easy way to preload maps of various types onto the device for occasions when you lost cell signal. many apps let you fool the phone into caching maps if you first view them while you have a signal, but that's a ridiculously inefficient system. I want to load plenty more maps than I'll need and go. I don't want to keep loading maps every time I go out. I just want to do it once and forget about it. Purpose-built GPS receivers can now take Google Earth imagery and more at the consumer price point, load them all on, and not require internet connectivity in the field for anything.

3: cell phones are still more delicate. at minimum you need to concern yourself with rain. ideally, you should concern yourself with extended vibrations on the bike and crashes, too, which forces you to invest in a quality housing (like an Otterbox or a Magellan Toughcase) and a mount. So the cost of the phone itself is not the only variable.

4: battery life. need I say more? you're lucky to get a couple hours of battery life out of a phone with the screen on constantly using GPS (and downloading maps). The Magellan Toughcase includes a supplementary battery to try to mitigate that, but my GPS receivers can last many hours all by themselves. My handheld even takes spare AA batteries and can easily be functional for days (with the screen on).

5: size. The phone already takes up more real estate on the bars/stem than an Edge. Add a protective case and it's even bigger. What about space for a light? I stopped using my handheld GPS on the handlebars because of space concerns.

There are lots of good reasons not to use your phone as a bike computer. Do phones have a place in the GPS world? Sure, I'm not arguing that. But I think their usefulness is pretty limited to use in the city (although there's legislation popping up now preventing the use of cell phone navigation in cars), geotagging photos, and the types of basic GPS functions someone might need or want while in civilization. I do not believe they are well-suited to backcountry use without extensive hardware and software modification.