Agreed, the arrival of the five 322s has covered the shortfall in electric units to operate services radiating from Leeds, procuring nine 317s to replace just these five units would be overkill. If the 317s also replaced the 321/9s as well as the 322s, then it might be a more palatable concept as nine 317s would then be replacing eight 321/322s.
Northern could use an extra 3 or 4 EMUs on the West side before any new electrification is complete. 3 would be sufficient but 4 would reduce the likelyhood of a diesel unit or combination having to fill in for an EMU.
Plus the Stoke services may be able to exceed the 90mph speed limit of the 323s, which would help with late running trains not holding up following services.