This thread is mainly based on the 717. The 717 is a great aircraft, and extremely efficient. It has a range of up to 1800nm (IGW version), so it can serve most destinations without a problem. However, it is being passed up by many airlines in favor of the A318 and 736 because of fleet commonality.
The A318 and the 736 are nowhere near as efficient as the 717. Yes, they have more range, but let's be honest, how many routes with a 100 passenger load require 2000-300nm of range? A large percent of all flights with a 100 passenger load are less than 1000nm. The 717 is obviously the most economical choice.
Airlines love commonality, but I think they are taking it a little too far right now. How much money does it really save the airline to operate the A318 or 736 b/c of commonality. So it all comes down to this:
Money saved through fleet commonality, or increased profits because of more efficient aircraft? What do you think?