In order to be measured you have to do something measurable.

Garmin units - without additional sensors - only measure GPS based information with regard to ride physics (cadence, speed, HR, etc)

When sitting still there is no GPS data change from which to draw metrics.

It therefore becomes apparent that in order to measure ride metrics one would need to employ the use of compatible sensors.

Many different sensors would work and this list is growing by the day thanks to the spread of ANT+.

Yes - Garmin's solution to use the GSC10

Powertaps can transmit speed and (virtual) cadence as well as power

Quarq's will as well - along with power.

As mentioned other ANT+ compatible sensors will also work.

For use on the trainer I prefer to also turn GPS off. GPS drains a lot of battery life. During the winter I can run for weeks without having to charge any of my Garmin units.

Without ANT+ sensors you can still use the head unit to track time and act as a stop watch for you. Probably one of the most effective features of the Garmin is the ability to program in custom workouts (intervals) and replay them allowing your unit to do all of the thinking for you while training indoors. This feature alone is worth the cost of the 500. IMHO.