At least finish the line said in full when quoting me, thank you. "but unless it can get cross-connected appropriately then it's just as bad as a shallow knowledge pool that exists only on surface level and never deepens, either."
The crux of the issue that I stated is having something being so broad in scope that it doesn't have any application to other research and not being able to see how it could be applied, as well as the inverse of something to narrow in scope that no applications for that knowledge can be devised not even a question to be answered (Fermat's Theorems being examples of such questions to be answered), are equally bad outcomes.
As for Legendre polynomials- Not in the least. Certainly, I personally do not understand all their uses, but I don't need to know them to understand that they were developed from a solid basis in mathematical physics and are useful in various aspects of physics but especially spherical harmonics (though they were not known by that name at the time) which were developed shortly after by Laplace, mostly to refine further knowledge of astronomy, specifically what we now call celestial mechanics.
Hyper-specialized knowledge that has purpose is very much so welcome. In fact, Legendre polynomials and all their assorted related equations and such are the perfect example of hyper-specialization that does, in fact, cross-connect appropriately within the many interrelated disciplines of the field of physics.