To be frank its a very complex subject. However as someone who has seen the effects of fleet commonality, I can tell you the cost savings are not to be sniffed at – i.e. several million ££s!!!
With regards to the benefits on the crew side of things, if an airline operates a fleet of 10 a/c for its short haul operation, regardless if its a single type or mixed fleet, the airline will still need approximately the same number of pilots.