FWIW, I've been told that it's much more likely to occur in the .45/10 frames (longer drawbars?), but I've seen it happen with an occasional 9 double stack frame.
The older trigger play spring (thicker and not connected across the top) was at one time made in a very shiny (almost chrome-like) steel. I only learned about it from a senior armorer at another agency many years ago. He said it was supposed to be stronger than the early spring that was copper-colored, and had supposedly been made to address the drawbar head/trigger ear "click" in the FBI guns.
He said it didn't have a part number, and I'd have to try to ask for some through just one person at the factory, as those springs weren't available through the regular CS/LE parts guys. He seemed to be right about that, as neither of the LE parts contacts in CA I knew (back then) claimed to know anything about those springs, and couldn't find any reference to them in their computers.
I called the person the other armorer had told me to call, and the first question the fellow asked me was how I'd even heard about them.
He chuckled when I told him, and then asked me how many I needed, so I asked for a few. That was quite a few years ago. (Don't ask if I have any "spares", either.)
However, I saw a picture somewhere online in the last several months, about a sale at one of the major parts vendors, and the trigger play spring shown was one of the bright, shiny ones. My guess (strictly just a guess), was that the company had decided to wash out whatever remaining supplies they may have found of that older spring, so they got dumped on the parts market.
As has already been mentioned, the gun will function normally without the trigger play spring. The SA trigger may have some extra "slop" in it, but that's not something normally noticed when you're shooting the gun at speed, for defensive roles. It's more for slow-fire target shooting, to give the SA trigger a little less "play".
As a matter of fact, I remember when a friend of mine with the CHP (weapons training officer) told me that the CHP had finally removed all the trigger play springs from their early 4006's. They'd tired of replacing the broken springs in their heavily-used guns, so they just drilled out the rivet holes so springs could no longer be installed (don't ask me why they felt they needed to drill out the holes). When they ordered their new 4006TSW's (and 4013TSW's), they ordered them without trigger play springs installed on the drawbars, from the get-go.
I didn't see (missed?) where someone explained the reason for the "click", but it's caused by the front ears (or prongs, if you'd rather) of the trigger rising just up and over the top of the drawbar's head. They normally remain in the wide V-notch at the back of the drawbar head, but sometimes, when the hammer is cocked into SA, the trigger can tip just enough to let the tips of the ears rise just up and over the top edge of the drawbar head. As the SA trigger is pulled, the tips of the ears, sitting on the top edge of the drawbar head's V-notch, are pulled down over the edge, back into the V-notch ... making the "clicking" noise as they snap down over the top edge of the drawbar head.
The trigger play spring can usually be adjusted to keep enough tension against the trigger so the ears don't rise that high, but the "fix" is often temporary. Also, too much, or frequent, "adjustment" of the spring can result in one side or the other breaking off.