Virtual power vs mph

You can certainly do it that way. People tend to be able to relate to power a bit better though. One reason will be factoring in accelerations. A better program will take these into account and not just relate it to the current speed. Even on a trainer accelerating faster will require more power.

Another reason (and likely the stronger one), is that the trainers are not linear. Meaning a change in speed does not always correspond to an equal change in power. A number of trainers try to approximate the resistance of air in this way. If it was linear, a change of 1 mph could mean a change of 10 watts (for an example). So going from 17.0 to 18.0 mph would be 10 watts higher. So would going from 24.0 to 25.0 mph. However since a trainer like the KK Road Machine ( and others) is not linear so that 24-25 mph change may be something like 20 watts because the resistance curve follows some sort of exponential function.

Should, but like a running treadmill there are differences to the real thing.  In the end it doesn't matter with VP as testing and gains are relative.  So as long as you are using the power vs. speed curve for the trainer you have and are consistent with the bike/trainer setup (psi, spin down time or clamping force, etc.) it's all good.

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What you're talking about is more or less how virtual power works.  Several years ago before I started coaching or had a power meter, I created the attached spreadsheet as both an educational exercise and allow me to train over the winter using the principles of power.

I would use the chart to determine the speed I had to spin the trainer to maintain various power outputs for my planned intervals.

Ben is right that the power curves for almost all trainers are not linear.

That being said, if you are focusing on steady effort intervals, you can just use speed to at least gauge progress.  For example, if you did a 5x5' interval workout where each rep averaged 18.0 mph (you hit the lap button on your computer to accurately record each lap individually), then the next day you shot for, and held 18.2 mph...then yes, you applied more power during the second workout, and you are seeing gains.  Maybe 4 weeks later you try the workout again and you do each rep at 18.3 mph which requires more power again...so you can correctly infer that you are making improvements.  How much improvements...as in how many watts?  You won't know that...but since you don't have a power meter anyway for outside use and racing...it wouldn't mean much to you anyway.

But if you are going on the trainer and just riding without any structure...looking at overall average speed may not be an accurate way to determine which workout was harder, or if you are getting fitter. 

I did the same thing for the KK fluid trainer then took it the next step and integrated Jorge's/BT winter cycling plan so back before the day's of me having ANT+ pickups I could do the plan with VP.

GAUG3 see attached.  I tried to match the curve published in the JFP manual as close as possible. 

Not a stupid question, an excellent realization.  Fortunately, you're not the first person to ask it.  Check out TrainerRoad.com, for a great tool that does exactly what you're looking to do.  Some trainers are more repeatable than others.  i.e 200w always = 20mph.  Others vary with temperature.  But yes- it doesn't really matter if 200w = 15mph or 25mph, as long as its consistent.  Welcome to the world of trainer power training.

you'll be buying a power meter in the spring.