Is mobile airtime free for life in the car hotspot ? What network does it use, on what tariff ? If it's free and never expires even if you don't use it, then I agree.
I use my phone as a car hot spot so the kids/missus etc can use their tablets for music, video etc. Works a treat, but eats a lot of data on longer journeys. Good news is it doesn't cost me anymore, as it's included in my £15 pm phone plan. If its free to do this using the car mobile connection that great, if not it "may" be cheaper to use the phone as a hotspot depending on how much data is used and the phone airtime cost.