When is it really necessary to go mafless?
When you don't have the equipment or knowledge to tune a MAF based ECU.
Often people unwilling to learn Motronic opt for standalone.
There are also race (or high boost) applications where Motronic will not cut it, and the only reasonable (or affordable) way to do it is with a standalone MAFless ECU.
There is definitely NO reason to use Motronic MAFless, IMO.