I didn't say otherwise. But consider this:
- Sheffield-Manchester Piccadilly (stopper) - you can get 158/170/185s from Sheffield to Manchester Piccadilly
- Chester-Manchester Piccadilly (via Northwich) - you can get 175s from Chester to Manchester Piccadilly
...by the same token I could point out that poor old LM only offer 350s from Crewe/ Birmingham to London (and ignore the fact that there's a faster Pendolino service). Yes, there are intermediate stations where the faster service doesn't stop, but I wouldn't pretend that all of these Northern services see many "end to end" passengers.
Comparing the two routes mentioned to LM's Crewe-Euston service. For a journey like Rugeley Trent Valley to London Euston you've got the option of travelling part way on a 350 and part way on a 390. For Delamere-Manchester you have to do over 80% of the journey on whichever rolling stock is running the Chester-Altrincham-Manchester service and for Hathersage-Manchester you could change trains at New Mills or Marple but you might not get better rolling stock.
Perhaps what the bigger issue with Northern rolling stock is that if you're making a journey like Southport to Bradford you can finish up getting Pacers for both legs. It's not an exclusive problem to Northern though doing Southport to Chester on Merseyrail gives you a longish journey on toilet-less stock with seating that isn't that comfortable.