On my trainer (KK Road Machine), trainer miles are a little bit harder than road miles. But that's not the reason I think running a speedometer is a good idea.

Trainer speed is related to power. For some trainers (like mine) that relationship is known well enough that you can "train with power" if you know your trainer speed.

If you have a good trainer (like mine) where resistance is not greatly affected by temperature, trainer speed will give a more reliable measure of effort than heart rate. HR is affected by heat (a real problem indoors), dehydration, even cadence.