I have to think you haven't quite understood how Bayesian filtering works. The stuff you're talking about (random words, transpositions) already makes an impact in your statisticts. In fact, it is better not to put them in the "correct" bucket, because as Paul Graham noted, where a spammer may try to subvert rule based filters with "vi.agra" instead of "viagra", the former will get marked as a 100% indicator for spam, where the latter might have been innocent. Likewise goes for random words.

As for the added complexity, it is not much complexity to add here at all. That's what's so appealing about it to me. There is no fundamental change in the way mail works with this scheme, as opposed to many others proposed so far. And I have a hard time following the argumentation that complexity necessarily makes a system easier to exploit. Taint checks make a program more complex, too. Encryption adds complexity, but I'm sure noone uses telnet for remote shells over the internet anymore. Complexity is not evil by itself - that's much too simplistic a world view. Everything should be as simple as possible, but no simpler (to invoke a well known quotation).