IIRC the D78s are proposed for the Harrogate Line amongst others. I don't know what the linespeed is along that line but I can't imagine that 60mph running would be a major issue along that route. Could they even go on Huddersfield - Bradford? I'm pretty sure that the 142s don't make it to 75 along that route and most of it's limited anyway...
This would be OK(ish) if these routes were operated in isolation, but the fact is they are not. Units operating on Bradford-Huddersfield (which is actually part of a bigger set of diagrams & operates on a route with some semi-fasts), Harrogate-Leeds (which might be electrified in CP6, but almost certainly will have VTEC services running along it bi-hourly from 2018), even Huddersfield-Sheffield at will also be used on other routes at some point. By utilising the slower D78s, Northern would have reduced options when rotating stock around to cover routes. So if, for example there are a shortage of units available for the Leeds-Sheffield via Moorthorpe, do they cancel or risk running 60mph stock along main line sections and risk causing delays for units that couldn't possibly keep the timings. Or worse still, do they alter the timings of all routes just in case D78 stock has to be used on any route? That's the big problem.
I've argued this matter before. Northern (or at least the Northern franchise) needs a flexible fleet that can operate pretty much anywhere. Sure there are currently exceptions from this like the 321/322/333 stock in Yorkshire and the 323s and soon the 319s in Greater Manchester / Merseyside plus some issues with other stock, but in large most of Northern's units can operate almost anywhere. It's how Northern has just about muddled through with a rapidly ageing stock, by being able to quickly move units around as required. But present them with stock that simply wouldn't be up to the task on a significant proportion of it's route and you are storing up problems in the future when the Pacer stock falls away, or is away for long periods being re-fitted.
Maybe there is limited demand for these cheap 'n' cheerful units, maybe. But it shouldn't simply be used in an attempt to cheap out, because sooner or later new stock will need to be ordered. And as the D78s will be a limited run, you will just end up with yet more microfleets and incompatible stock. Just what the network as a whole doesn't need. Better to bite the bullet now, make arrangements with one or two suppliers for new, compatible stock that can suit the needs of all TOCs that are going to need DMU stock 20-30 years from now.