First the term. By fragmentation I mean two things: 1) we have many libraries doing the same thing. 2) the implementation is JS is not the same in every environment.
If you are spending most of your time writing other languages which have a standard library, I can understand your opinion.
However, I see fragmentation as:
1) It introduces the opportunity everyone on the planet to give a shot at implementing something that may or may not be better than we consider today the best. For example I used a datepicker in my latest project but in the current it failed and I could replace it in 15 minutes rather than spending hours finding the issue with the "standard one". I'm not really experienced in the C++ world, but I guess people would call me crazy if I proposed a new stdio lib. Maybe there could be better libraries, who knows. It's a settled game there. See jQuery in JS land, it was for many years, "the golden tool". Now we have alternatives for more specialized workflows. Not everybody wears the same hat all the time.
2) EcmaScript is constantly evolving, that causes another fragmentation, but this also allows the dev community to propose changes, implement new features and create a really vibrant feedback loop. If you stick to the latest stable (currently ES5) you are safe to build whatever you like with great stability.