The difference between the Birmingham-Manchester and Birmingham-Liverpool services is that many of the "Liverpool" passengers change at Crewe for other routes.
Agreed, however I use the service Birmingham-Crewe (for Wilmslow) and Crewe-Liverpool (from Wilmslow) and both seem busy.
Vastly preferable getting on the service at platform 4C in plenty of time from Birmingham, compared to waiting for the XC service, unsure of getting a seat, and changing at Stockport.
For me it's a good service, reliable, and I don't miss the lack of onboard catering.
Oh yes, and splitting tickets at Stafford gives me more permitted routes from Birmingham (Lichfield and Tamworth if I want) and First Class open return cheaper than Standard Class through ticket before 09:30.