Bradford, Halifax & Rochdale probably provide more passengers for Manchester Airport than Chester & North Wales and also need the links to Piccadilly provided by the Ordsall curve.
I certainly agree with the first bit. I'm a bit vaguer on why York needs 2tph to the airport with identical calling patterns, although I'm not even sure if that's intended to remain long-term.
I think the fact that Newton-le-Willows and Warrington will have Airport trains anyway counts against direct access from North Wales.
Having said that both areas should have direct services. Could North Wales services travel via Crewe instead of Piccadilly, or would running right across the WCML cause too much disruption?
A train from Chester going towards Manchester Airport would be much like the current Cardiff-Manchester trains, except that they would only be able to use platform 6 in either direction (rather than having a choice of 5 or 6), and would reverse there.
If you diverted the North Wales-Manchester trains to Manchester Airport via Crewe and Wilmslow, it would need two more 175s and release one 150 currently used on the Chester-Crewe shuttle.
The new Chester-Manchester Picadilly trains would have to leave Chester 3 minutes later and arrive there 3 minutes earlier than at present to offer a valid connection at Chester. The westbound train currently joins the WCML 5 minutes after a Virgin train, so this would eat into the margin and remove valid connections from Scotland to Wales. There's more slack in the eastbound train's timetable.
I feel such a move could upset some people.