So it's nice to have my prejudices confirmed by real, live, published scholars! Here's Peter Jona Korn* talking about the Second Symphony of Roger Sessions, which is organized atonally but nevertheless uses key signatures:
A minute analysis of this symphony will no doubt confirm the existence of carefully submerged tonal centres, to which these key signatures have reference. The same kind of analysis would bring forth very similar results with any given score of Schoenberg or - much more so - Berg or Dallapiccola, none of whom uses key signatures. Atonal music is, after all, nothing else but tonal music in which the tonal functions occur and permute in the shortest possible space of time.The Symphony, ed. Robert Simpson, vol. 2, pg. 261.
Ha ha HA HA! Told you so! With both Hindemith and Peter Jona Korn on my side, the world will be at my feet in no time.
*Who? Apparently, a German composer (1922-1998). Studied with an impressive list of composition teachers (Rubbra, Wolpe, Schoenberg, Eisler, Toch, Rozsa, and Dahl) and had a successful academic career, but his music is almost entirely unknown.