The focal length stays exactly the same as written on the lens. It does not change by changing the image format. So that 55 would be a 55, and so on - on APS-C, it has the same angle of view as a 85mm on FF (or a 150mm on 6x4.5cm)...
If you own small format lenses of identical focal length, speed and generic type, you might not want to bother. The small format lenses will usually be just as good, but much less heavy. Medium and large format lenses usually have quite similar lp/mm and contrast specs as any equally professional small format film era lens - the benefit is in the extra coverage giving higher contrast and resolution over the total larger image. Which you don't make any use of if you already crop the centre 24x36mm (or, worse, APS-C) in-camera.
That said, it is perfectly valid to use medium format lenses wherever you have no small format equivalent of them. I sometimes use a Mamiya to Nex adapter for the Mamiya macro, shift and soft lenses - but after I got the A7, I quit doing that on the Nex, the area loss is a bit too big there, and haze issues (due to the vastly excessive image circle) were frequently occurring especially in macro.
