Snam,
Indeed the 737 or 767 transition is cheaper than a full transition course, but also the 320/330 or 340 CCQ is cheaper than hiring a 737 DEC to the 330 !
Also it would be cheaper if IM and BW served on the 320 as they were current on the 320 (fleet that is short of instructors) it would be cheaper and more productive for QA if they served on the 320 for a while before they get their CCQ 330 as soon as they joined.
Of course I don't mind if they did that, what I really mind is the use of the "cheaper" excuse just to hold a group of people flying a hard roster for a long period of time while others just jump to the safe haven of the nice destinations and better life style.