If shorter vehicles existed, they could be used instead. The chosen vehicles tend to be selected from what's available at the time at a suitable price and possibly also with suitable operating characteristics (e.g. top speed). And what the others have said.
...and of course recent conversions (at Eastleigh) using vehicles from the likes of 508s are using stock that would otherwise have been scrapped, so must be one of the cheaper options.
It's a rather nicely engineered solution those 508-based EMU translators. Even have integral tail lights fitted. The cab ends are used to couple to the dragged EMU, clearly since they already had compatible couplers
Theoretically yes but not really. Two Mk1s vs two vans at, say, half a Mk1 length 'saves' one vehicle. When not actively being used they tend to sit in places where there is enough capacity.