It depends which diagram needs covered - at present only Piccadilly and Lime Street sign 323s, whereas most depots sign 319s, so it's normally easier to pull a DMU from another diagram to cover a failed 319. Also there's the time issue of getting a stepped up unit to where it needs to be, so in the case of a Liverpool area failure a unit can be quickly nabbed from a Cheshire Lines diagram.
Oh yes, I do of course appreciate both the above points. On the logistical issue, don't forget that there is so much ECS running at the moment that getting a unit from A to B doesn't necessarily mean physically doing that, the same can be effectively achieved by refraining from sending one ECS the other way. For instance, the 0729 Alderley Edge to Piccadilly is covered by the unit off 5H43, 0540 ECS Allerton to Alderley Edge. (This is the unit which parks at Stockport for the rest of the day). Clearly, it would be as easy to get a unit from any NW base to Alderley as from Allerton. Whether a unit would be available, and whether co-operation between bases exists to the required extent, is obviously another matter.
On the 319 v 323 issue, it would be easy to counter that only Piccadilly and Lime Street crews currently operate the full Chat Moss route (I presume?) but I believe there is some interworking of 319s between Chat Moss and Lime Street to Wigan/Blackpool services, which I see as regrettable, if the 319s and 323s aren't going to be fully interchangeable.