David--you're right about split rigs reducing crew sizes. That's the basis for both ketch and schooner rigged boats. It is also a good way of getting lots of sail area "low" which works well for heavy displacement boats, too. On the matter of yawls it is a bit different. The design was favored for a singular reason (by S&S especially) and that was to beat a CCA racing rule. The rule said that sails set on a mast that was aft of the rudder post don't "count" as sail area in the same way as sails set on a mast forward of the rudder post. Therefore, the yawl came into favor.
I know a man with two 44 ft Naval Academy yawls (designed by Luders) . He wanted to install an engine in one so he unstopped the mizzen mast to place the engine below. Famous America's Cup sailor Dennis Conner said the mizzen acts like a brake on that boat. So he took off the mizzen but he kept the other yawl as it came. It ended up that the first yawl (now a sloop) became the faster boat. You can read about it (see page 3) on this .pdf document just look for the story about yawls Flirt and Frolic http://www.amss.us/sitebuilderconten...albapril09.pdf