I stand by MotorVac as a periodic service.
Here is a good test.
Have the shop pull and examine the plugs before and after the service. Or even have the tech "finger" test the carbon deposits through the plug.
Also, if you can manage it for free, get a pre-motorvac gas EMISSION report for the NOx and what not.
Run the motorvac.
The results speak for themselves.
Now for ROUTINE vs. Periodic maintence, yes, Nuetra, Redline S1-1, Lucas UCL, and Chevron Techron are amazing system cleaners.
Many here use Redline and Techron every 3,000-6,000miles.
BG44K I personally classify as a Periodic treatment due to its aggressive chemistry.