Is there any use to motor pacing for people in the cat 4 to cat 2 range actually useful?
Background

If you have the resources (motor scooter and someone experience in how to do it) and time, motor pacing could be very good training tool for building high-end power for a variety of fitness levels. Where it excels (when done properly) is getting riders to train at their lactic threshold. Motor pacing will push you to levels that are hard to replicate outside of real race conditions.  

The consistency of motor pacing is also great as you can fine tune the intensity. The fine tuning allows you to sit closer and longer on your threshold than might be possible with a group of riders, especially if your group of riders has limited riding experience and differing fitnesses. Motor pacing can also accommodate a variety of fitness levels, the driver simply drives at the appropriate pace to put the rider at their threshold.  

Now will this be the "secret sauce?" I have some doubts for the following reasons:

1) Drawbacks for inexperienced riders / clubs

Cat 3 can represent a wide array of abilities and skills depending on your region. Many newer riders do not know what it is like to ride at their true threshold.  They hold back, resist the wonderful lactic acid pain rather than embrace it.  For them, motor pacing may have fewer benefits over regular fast club rides.

Inexperienced riders also may not have the riding skill to motor pace safely. To motor pace well you have to be able to ride with your front wheel an few inches from the tire of the lead vehicle. This can be dangerous for unexperienced riders who may suffer from lapses in concentration.

Your diver must also be experienced.  I have paced behind an inexperienced driver and it was frightening to say the least.  Their speed would surge and slow and at one point I actually hit the vehicle with my front wheel.  I didn't crash, but this could have bee some serious carnage, especially for the riders down stream of me. 

2) Raw Power Rarely Wins Races

Very few riders win races by riding off the front with their superior power. If you see someone riding off the front of a pack it is because the pack has allowed it, it is not due to their 'superior' power.  Very few riders ever ride away from the group through pure strength. Ones that do are typically so naturally strong their rarely stay long in Cat 3 anyway and in few years will likely be riding pro.  

Not everyone has this genetic ability, and its also not the only way to win races at the local level. Strategy and race smarts can be of equally importance. I find many new racers overvalue fitness and undervalue learning how to read races and plan efforts. Too many are too twitching looking to jump on and 'go fast' too early in the race. For these riders motor pacing will not teach them how to read races.  It will not teach them how to make a plan, and commit every last ounce of energy to it's execution (i.e., execute, or fall in the ditch in exhaustion).  It will not teach them how to neutrally deconstruct a race and assess what worked and what didn't and come up with revised strategies to try the next race. It will not magically impart singular focus and the passion needed to win.

That said, motor pacing can still be a frickin' blast when done right!