I didn’t mean to imply that there are no uses for these Dash Buttons … they definitely seem appropriate for children due to their robust form factor.
However, the point of a $5.00 button is to save money because you are using them in quantity – quantity sufficient to justify running the server as well (yes – a few of us have servers running, of course, but … not everyone!).
But there is one point not mentioned: These are pretty insecure. MAC addresses can be spoofed. I wonder if these broadcast their MACs at any time before being in WiFI encrypted mode.
So while I can think of 1 or 2 uses for them, that’s too few when a relatively cheap alternative is available. I’m not sure what minimum quantity would be worthwhile.
For the “Presence” case, car or otherwise, consider my beta SmartApp which would enhance security by requiring you to not just press one Button, but a sequence of Buttons (a PIN) of as little as, well, 1 or 2 digits, frankly. This gives you 2-Factor Authentication for arrival presence (the unique secure Z-Wave Aeon Minimote Device, and the PIN sequence you must enter, making the Device unusable by intruders):