I suspect the main reason people will buy the straight spine mule is that they want to make their own knife, not that they want to get into comparative steel testing. The S.S. mule is to always be in stock for whenever you get the urge to put handles on a fine slab of PM steel and call it uniquely yours. I have loads of mules (even the standard run SPY27 mule) and I still bought a S.S. mule because it's a safe blank on which to test and modify my own designs. If I screw up, I can get another for a modest layout.
I think Spyderco noticed a lot of people were enjoying the process of customizing a knife, rather than engaging in rigorous testing, so decided to fill that vacant spot in the marketplace. I'm guessing that if Spyderco had continued to produce the leaf-bladed SPY27 mule, you'd have had upset collectors complaining.
I'm liking the 'difference' of the S. S. mule...the subtle change to the spine makes a big difference at the tip. It's a real cat's claw, and can start cuts in tiny places.
If Spyderco continues to pursue the "I want to customize my own fixed blade" market, I wonder if they'd ever consider a wharncliffe or sheepfoot design.