At the expense of short forms it seems Northern can get away with very few in service, you would have to ask why they would want to fill Allerton, Springs Branch, Southport Platforms and anywhere else these junk Frankensteins spend their time with even more of them! In the not too distant future they are going to be surplus to requirements anyway, thankfully.
Also they already have a dedicated team at Allerton, they have specific special diagrams that means they are cycling through the depots every other day and still they are broken every day.