I'd also like to know what people think about the issue of whether Boulez's conducting career has taken too much time away from his composing.
For what it's worth: I'm more of an admirer of Boulez as a conductor than as a composer, so I guess my answer is no, although if his conducting activities hadn't taken off in the way they did, perhaps his music would have retained (what I see as the) structural ingenuity and expressive originality of Le marteau sans maître rather than becoming increasingly long-winded and decorative.
That's more or less identical to my own feelings on the subject.
I think there's a further issue (much as I admire his conducting) as to whether his influence as a conductor and programmer has been primarily for the good. That's a question I'm less sure about; I do think he's stopped some good music from being performed ... No doubt Oliver will be along shortly to say that that's inevitable and not blameworthy, which I suppose is true, but I still think it's relevant, especially if we're considering what the musical landscape would have looked like if Boulez had composed more and conducted less - paradoxically it may have meant there'd be a stronger Boulez oeuvre but a less artificially cordoned-off and venerated one.
That other r3ok favourite Adorno makes much of Mahler's conducting as a form of alienated labour, taking him away from his 'proper' work of composition (although he also says that Mahler's experience as a conductor is a highly significant factor in the music, often against the grain). Did Boulez ever have any financial need to conduct?
I'm not aware of it if so, but it's an interesting question. I've tended to assume that his conducting activities in more recent years have been a cover for the composing he knows he wouldn't have got round to anyway, but that doesn't really explain why he
started doing all that international conducting, back in the 60s and 70s (or maybe it does).
I thought I'd read
A Musical Physiognomy, but I don't remember those comments about Mahler's conducting activities at all!

Boulez says somewhere that the risk for a composer who conducts is that his orchestral music turns into an exercise in what's possible; that he gets to know conducting and orchestration so well that he never writes impossible things and leaves it to the players/conductor to
make them possible. I suppose the attempt to counteract that is what you mean (what Adorno means) by "against the grain"-ness?
Incidentally,
"open-ended structures and mania for revision" (I know it's not your phrase, Simon!) seems to obscure rather than clarify a significant issue, which is that at some point Boulez's interest in openendedness seems to flip over into an interest in perfecting an (albeit in some cases almost endlessly deferred) final text. Revision as a multiplication of versions isn't quite what he's up to these days, is it? The revisions now replace what went before, which is very different from the Third Sonata-period open form aesthetic and only quite superficially relatable to it.