To me it looks as if we are trying to 'square a circle' here.
Airlines basically only want a 'bum on seat' who can operate to an acceptable standard in a multi-crew environment, follow SOPs, walk without a white stick
, acquire sufficient hours to be eligible and able to pass the command course and land the a/c single crew if necessary.
The issue of WHY single crew ops are 'good for you' is that they teach you AIRMANSHIP. Something that can take years (or for ever) in a multi-crew, SOP bound operation. I did 5+ years of single-pilot ops between RAF and airlines, and they were the hardest civil flying I have undertaken. Without my RAF experience I would have been 'sorely tried' on more than one occasion. It is a great training ground for learning to keep your skin intact. It is so much harder and takes more overall flying skill than 'big' airline flying.
HOWEVER, no, it is NOT good training ground for multi-crew airline ops. It is all down to what you personally see as a 'nice to have' in the flight deck. No ab initio 'cadet entry' pilot is, by definition, 'bad'. He/she will just be less able to work out that 1 in 10,0000 solution which is not covered by BA, Aer Lingus, Air France, Lufthansa, BMI SOPs. My own feeling is that when the 'chips are down' your 'average' ex single-pilot operator will fare better than your 'average' ab initio. Whether or not the s/pilot operator will be able to adapt to multi-crew - well, we hope so, and should be able.
Doesn't make ex s/pilot ops
a) Necessary
b) Good for crew ops
c) Always safer to be with
.......but I reckon it is invaluable experience.