You bring up some excellent points I think, and 'hit the nail on the head' regarding the smart phone cost.
I do not have a smart phone and would not consider getting one to use primarily as a navigation device due to the ongoing costs ("the Plan" as they call it). Now as a lot of people (most people?) today seem to have a smart phone anyway, your point becomes a lot more valid. Still, it is not a perfect or even the best solution to navigation duties on a motorcycle IMO.
The first major problem is that the screens of virtually everything made today, with the exception of motorcycle GPS's, are not bright enough to view comfortably or well in direct sunlight. Sure it may be good enough but a true motorcycle GPS has a bright, high contrast screen that is just about impossible to replace; and I have been trying to replace them for years.
The other two main duties of a GPS on my bike are to provide audio (music, audio books, spoken directions) and a phone interface. Obviously the cell phone being used as a navigation device, mounted in front of the rider somehow, would function as a phone too but I do not know if it would be as good or easy to use as a motorcycle GPS. The display shows the caller ID and big icons for Answer and Ignore and these are easy to see while riding down the road. As far as audio, I suspect a smart phone could be coupled into a comm. system on a motorcycle easy enough.
Two more issues are weatherproofing and charging; the dedicated motorcycle GPS addresses both very well. They are weatherproof and can simply be ignored and left in place in a driving rain. A smart phone would have to be removed, covered with something like a plastic bag, or put into some type of weatherproof case, these last two decrease the already marginal visibility even more. Finally, if one rides for a long time, and some of us do, a mounted GPS will continue to function and remain fully charged as long as the bike runs. Not sure what it would take to charge a smart phone underway although of course I believe that could be done fine. But the charging connection would not be waterproof, unless the entire unit was mounted in a weatherproof enclosure to begin with and that is usually a big pain to install / remove from the bike when getting on / off the bike and wanting to take your phone with you.
Another thought that probably does not apply to most people but some of us really count on our GPS's while riding and the best way to ensure success is to carry two. I carry two GPS's when traveling reasonably locally, and three when traveling long distances. That is just not practical with smart phones for the obvious reason, cost.
All of that said, motorcycle GPS's are tremendously expensive for what they provide. Unfortunately they are about the only way to get what they provide, mostly that bright, high- contrast screen.
Brian
So does your reply mean that the Garmin does not offer anything over a smartphone?
Of course then the Garmin would be useful for those who do not want to pay monthly for the data plan, but if someone already had a smart phone, it would make the Garmin obsolete.
Or is there a benefit to Garmin that I'm missing?
I didn't realize that so many people had expensive motorcycles but didn't have a smartphone.
