I think McKenna served his purpose well: to convince many intellectuals and academics that drugs are okay to talk about, even in a scientific way. Anyone who is a "legalize-it activist" is okay in my books. Even if they're a fucking drug lord or something.
But yes, he probably got that brain tumour by shoving huge amounts of a huge variety of natural and synthetic hallucinogens into that poor over-burdened brain...
Hey, he got famous because he was incredibly articulate, well-travelled, but above all, radically new and original in his thinking.