Are they relevant in the real world, especially when using IDEs?
Maybe you could elaborate a bit on what you mean by "know namespaces"? If you just mean particular sets of variables or functions (because they're relevant to whatever that programmer is doing), my answer's probably "no" because these are the types of things that are trivial to look up. If you use them enough, they'll probably get memorized (if that's what you mean by "know"), but I don't think it's necessary or important.
(if this is completely off-base, elaborate a bit and I'll revise).