Should mileage on a trainer be considered mileage on the bike?
Most bikes I've looked at (and purchased) via craigslist and other online listings don't usually state miles, just condition. I think most bicycle owners don't have odometers.

The mileage I keep track of is more for my own knowledge and maintenance scheduling. Keeping track of when I change the chain, or how long tires last type of thing.

That said, I won't get another sensor that only works on the front wheel, as I like the feedback (speed, time, mileage, cadence of my workout) when I'm on the trainer.