I think it'd be a good idea, simply because we need more direct services from Birmingham to Manchester. All we have right now are the XC services, and we all know how bad they can be!
A half hourly service between Birmingham and Manchester seems entirely adequate to me, the problem is the length of the existing trains but unfortunately nothing's going to change that any time soon.
As such I can understand your reasoning from a capacity point of view, but I'd prefer it if no more services were forced through the Birmingham - Wolverhampton corridor, there's absolutely no opportunity for service recovery as it is since the XX:21 Birmingham - Crewe LNWR service was introduced, meaning that the Manchester trains are often delayed in their wake. At present leaving New Street northbound on the Stour lines there is:
XX:15 Edinburgh (Avanti West Coast)
XX:21 Crewe via Stoke (LNWR)
XX:25 Shrewsbury/Aberystwyth (TfW)
XX:31 Manchester (Crosscountry)
Which leaves no margin for error at the slightest delay. The XX:57 Manchester departure from New Street on the other side of the hour seems to fare better, services are a bit more spread out in the 20 minutes before that departure.
I must admit that the XX:21 LNWR services have been useful to take pressure off the XX:31 Manchester Crosscountry service though, and I frequently use them now even though it requires a change at Stafford or Stoke onto the following XC service to get home after work.