This is my own personal (and uneducated) opinion. It seems that with the factory recoil spring assembly, the small inner spring doesn't come into play until the last inch or so of travel. I can feel it in my slide, and it's a different feeling than the binding you get in the last 1/4". You can rack the slide back and towards the end, it feels like an extra "cushion".
My opinion is that the dual recoil spring assemblies are designed to reduce the effort it takes to rack the slide by making the "primary" spring (the larger outer spring) lighter. Then the "secondary" spring (the smaller inner spring) can be made much heavier to help alleviate frame battering from hotter ammunition loads. The full-size M&P and the compact M&P pistols don't use dual recoil spring assemblies; they use conventional single recoil springs. I believe that the smaller nature of the Shield is what compelled Smith to use a dual spring design in these...again, to handle the wide range of ammo that could be used. The full-size and compact M&Ps have larger slides that can be easier racked (because of the increased purchase on them), so the need for a light spring for slide operation isn't quite as critical there.
Dual recoil spring assemblies don't exactly have a perfectly clean reliability record. Many people attribute problems with 4th generation Glocks to the change to a dual recoil spring assembly (from the 3rd generation's conventional single spring). And just from my very short time here, I've read countless accounts of Smith replacing the recoil assemblies for Shield owners (and sometimes the replacement springs are not 100% either).
I'm kind of surprised at the reception this has received. This board in general seems very quick to recommend Apex trigger parts to improve the trigger system, even though the stock parts work just fine. And Apex parts are proven; I get that. I think that with some more time and experience, folks will see that the dual recoil spring design leaves a lot of area for improvement (or at least optimization for each shooter's situation) and this will eventually become part of a "gotta do it" modification list like the Apex trigger parts seem to be.