I've not done the maths, but would be interested to know if the ones from LM would be enough to convert the Manchester Victoria - Eccles - Liverpool, the Liverpool - Warrington Bank Quay and the Manchester Airport - Liverpool services?
There's not much scope to use them on other Greater Manchester routes (given the lack of paths caused by VHF etc)
(the only three services that could convert from diesel to electric under the current plans and current timetables)
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Hardly a 'northern thing' though - most 1980s/1990s electrifications received second hand EMUs which weren't occasional much of an improvement on what went before them. 305/308s/310s on Liverpool St-Cambridge's, 304/308s on Birmingham CrossCity come to mind. Makes the sums add up better, unless you want to write another cheque out on top of electrification works.
Sorry, I'm not suggesting it's exclusively "Northern" - I can see the logic in using cascaded units, it keeps things cheaper in the short term (after the expense of wiring etc) and it gives time to see whether three/four coach EMUs are needed in the longer term (before ordering them).
TBH I'd suggest the same thing if the Cardiff Valley lines are ever wired up, or the Merseyrail extensions etc. I'd take any old EMU over a Pacer round these parts!