What size motors are you looking at? What type of environment are the motors in? What is the importance level of these motors? All of these are relevant questions. I would not spend much time on 30HP and less motors unless they are vital to the process. If your tester requires you to take the motor off line and disconnect the leads and then run the test, I would have to look at that amount of time versus just replacing the motor on a regular basis or waiting for a failure.Preventative and predictive maintenace on motors is still not an exact science from all that I've looked at. I have seen all kinds of companies that sell all kinds of systems. I haven't been impressed with many of them. At one point a company came in (when I worked at a gov site) and tested all of our 4160 volt motors. When I looked at the "engineer" who was running the test and their test equipment, I noticed that it was only a 250 VDC meggar. He hooked the leads to the motor at the main breaker (no starter) and had them on for only 1 minute. From that he gave all kinds of stuff like DAR, PI, waveform, etc. The only problem was that to get some of these numbers (like a Polarization Index - PI) the convention is the 10 minute meggar reading divided by the 1 minute meggar reading. When I question them on this, first they said that the computer model extrapolated the results then the changed their mind and basically said that their PI is not the same as other PI. Also when I question how effective a 250 v meggar is on 5 kv insulation, I was told that their computer model extrapolated the results. Unfortunately, I threw out the BS flag on that one. It turned into a big argument and I was labeled a troublemaker from that point on. They made lots of money off the taxpayers, recommended that 25% of the motors be tested again in a year and left. I was pretty sure we had just be sold snake oil.
I guess my extremely long rant on this is to let you know that some of what you see out there is really no better that putting on a meggar and looking for 1 megohm per kv plus 1 megohm (hence the 1.48 on a 480 volt motor). Look at vibration and infrared (even though it is rarely cost effective to actually purchase this equipment since many companies offer these services extremely reasonably priced). Also look at how you are actually using the motors. A few minutes spent on a good design goes a long way (like on VFD motors and cable length)
Also, higher voltage motors lend themselves to much more testing than 480 volt motors (ie doble PF testing and dc hipot testing). I have had much better luck on these than the 480 volt motors.
I know that I really didn't answer your question, but if you give a little more information and I will speak up on my past experiences (even though they may not be the best).