Re: XKCD has been totuting the complexity angle for years
Okay, but how many of those 300 words are you going to be able to commit to memory - bearing in mind that the average number of discrete items the human mind can hold onto without some 'story' chunking them together is 7 +/- 2.
it's a nice idea, but I don't think you're gonna memorise that many random words as easily as you're gonna remember to take the first character of each word in the phrase, transform them, plus add on something that the site/service itself reminds you of. I appreciate the maths and all but real human beings aren't calculators or Mr Memory and there are good reasons why Bruce Schneier recommends doing it the way I have suggested and not the xkcd way.
Passwords aren't about pure Maths, they're about imperfect human beings.