It basically comes down to one fairly simple question: do you want authoritative, detailed knowledge of the language, or are you happy knowing it well enough to use it, realizing that there are probably parts you don't know well, and quite possibly other parts you don't know at all?
Basically, there are some people who seem almost born to be "language lawyers" -- they won't (can't) rest as long as there are even a few nooks or crannies in the language that they don't know and/or understand. Many of these gravitate toward relatively simple languages simply because learning more complex languages at this level of detail is often next to impossible for any one person.
For most programmers, that would be an utter waste of time -- they're quite happy and productive knowing enough of the language to be able to use it (reasonably) well. Beyond that, they simply don't care. The language is a tool, and as long as they can use the tool well enough for their purposes, the fact that there are other things they don't know is largely irrelevant.