That may be necessary if the next Doctor's appeareance was a surprise, this time we all know how he looks like. It's the reversed of Colin Baker's. We know the end result, we just don't know the road there.But for a proper regeneration you need the start and end. While not always was the previous actor always available (Colin baker most famously) they always managed to show both an in-process previous doctor and the resultant next doctor. The only regeneration to happen completely off screen was McGann to Eccleston. And it simply wouldn't work to have the doctor start a regeneration only to never see the result. Unless of course the result was a dead doctor that as a result of his final regeneration dissolved into nothing.