Testing a new technique in the heat of battle could result in death. If I was in an environment like that, then I would test any new technique on an enemy prisoner with the comfort of knowing that I would have my boys with me to help pull him off when the technique fails.
Interesting speculation, although at least in my shaky knowledge of history, I've really only heard of that practice anecdotally in one culture among one warrior-elite social class famed for their swords...
I think too, that it's generally believed that, at least among the mainly empty hand styles which the majority of us seem to primarily train, that they were designed and evolved not for the "heat of battle," or with access to "enemy prisoners," but rather among and for civilians more for, as you first speculate, robbers, muggers, and general noisome brigands and ruffians...
I would also guess that nowhere in the span of quiiite a few generations has there been an individual to have truly tested
all the possible iterations and applications of even the simplest of art forms. The premise, the overall theme of the art sure, but not all the material. The amount of altercations that would require is immense, even if we were to imagine each iteration occurring only once, which wouldn't really be a strong base of knowledge.
No, I think it quite likely that even among the proverbial "Old/Ancient Masters," the vast bulk of their practical knowledge came from controlled training and speculation, as well as knowledge handed down and accepted largely by faith, though tested in training. Just like today, I imagine that those much revered guys of years gone by probably hadn't used most of what they practiced and taught on the "streets" of the days...
Might they have used it more than the average First World Suburbanite of today? Of, definitely. Sure, undoubtedly. As far as the strategy and overarching premise of a style, I'm sure they had more hands-on experience, by and large, but in terms of having used each technique? I might guess it was only slightly more than the average slightly-belligerent, risk-taking martial artist of today...
I'm surprised Chris Parker hasn't jumped in yet, this seems like his kind of thread, and I'd be interested to read his viewpoints on the subject...