Having gone from single pilot to multi pilot operations and back several times I can kind of see their point. Jumping from one type of operations to another for a relatively low experience guy can be an issue. Going back to multi crew from a period of single pilot ops requires a bit of practice and simulator time, but is infinitely easier than transitioning to single pilot from multi pilot. If you need captaincy time it would be easier if you got it first then progress onto an airline job where it will be required for promotion. Or am I missing something?
Fundamentally, I would prefer to train a competent single pilot IFR guy on multi crew operations than try and teach captaincy skills to someone who has almost no solo time.