The initial ones are, but who says there won't be more if they are successful? They would provide a useful enhancement of capacity, allowing a reduction in 2-car operation which in my view needs to die out other than on a few branch lines, as well as reduce the need for under-wires diesel operation.
You've hit the nail on the head. Two coaches is okay on a rural line outside of peak times and holiday season. That's all it's good for in the late-2010s
For example at Heaton Chapel this morning;
Chester to Manchester: 2 cars and rammed
Buxton to Wigan: 2 cars so I get the Crewe to Manchester which is 3 cars (should be 6; why don't Northern double up 323s à la London Midland?)
Hazel Grove to Manchester: 2 cars and rammed
Buxton to Clitheroe: 2 cars and rammed
A normal 319 could work Stockport or Hazel Grove to Piccadilly stopper and back all day without the need for extra electrification thus freeing up one Sprinter to strengthen a service from 2 cars to 4
Therefore, scenes like the beginning of last week when trains roared through stations they should have otherwise called at due to a lack of coaches would become history.
To stay perfectly on topic, a great example for the Flex 319 would have be the morning Crewe to Bolton prior to electrification of the line north of Manchester, as it would only have to use diesel for a slower part of its journey and freed up 2 Pacers (line speeds south of Manchester are higher than those onwards to Bolton). However, by the time the Flex 319s are ready, this stretch will have been electrified
(In Northern's defence, in evening peak, the 17:23 from Piccadilly to Hazel Grove is actually a 323 and not a 2-car Sprinter, which I think is the only time an electric currently visits HG?)
there is a short period between March and May where Blackpool N will be open but still Diesel.