I don't think Edison was ever a patent clerk; I think you are confused about that. (Einstein was a patent clerk, if that's what you're thinking of.)

But Edison certainly did pay attention to what was being patented, and maintained his own miniature patent library at his laboratory in Menlo Park. Many inventors pay close attention to other patent filings, and indeed the entire point of patent filings is to give publicity about the invention in exchange for a temporary monopoly, under the idea that the publicity will spur innovation. What Edison and his employees would do is look at inventions patented and look for good ideas that weren't quite there yet — ideas that seems promising but the inventor hadn't figured out how to make them work, or had missed something important, or just hadn't thought it all the way through. Then they would see if they could remix that idea and get something novel out of it.

This isn't copying. It's innovation. We have a myth about how innovation works, in which geniuses come up with new ideas wholly unthought of, and change everything in a single invention, etc. It's nonsense and bears little resemblance to the history of science and technology, in which every idea has an antecedent and there are always at least a few smart people thinking along very similar lines at any point in history. That doesn't mean that they all have exactly the same idea — sometimes the one "genius" is the one who takes the idea just a step further than the others — but it does mean that our sense of how ideas work is generally wrong, because we don't teach people about the other people who had very similar ideas (you probably know something about Einstein's ideas, but you probably have never been taught about Poincaré's work in similar areas, and the differences between Einstein and Poincaré that made Einstein Einstein and Poincaré Poincaré, but historians look at these things).

So does attention to patent filings matter in the history of invention? Definitely! There are many examples of this, and any firm today of sufficient size keeps close eye on the patents of their competitors. Is this "stealing"? No. It is 100% legal and encouraged — it is the entire point of open patent filings. Does it mean that our idea of what kind of "geniuses" inventors are is frequently erroneous? It does! But not for the reason you're saying — it's not because they're bad inventors, it's because invention doesn't work the way we like to mythologize it.

Separately, with Edison, many of his patents were patents taken out in his name by employees. This kind of corporate patenting is not uncommon, though Edison's approach — assigning them to him personally — is unusual. But it was part of his mystique, building himself up as a "lone inventor." Is it nonsense? Definitely. That doesn't mean that Edison didn't accomplish incredible things. Historians are frequently more impressed with Edison's work as a system builder than an inventor — the lightbulb wasn't such an incredible thing by itself (his was just an economical variant on an idea that had been tried many times before), it was the fact that he used the lightbulb to sell people on the idea of electrifying their homes, which built up the modern electrical network. That's the part of Edison that made him so dominant, in the end — he didn't just invent things (or pay people to invent things in his name), he actually got those things into every house in the world, and that's the hardest part of technological innovation, it turns out (most patents are never commercialized, much less turn into ubiquitous objects in our lives).