It could well have something to do with that in Safari, the words you are typing are more likely to be words that auto correct doesn't recognise, like passwords, URLs, addresses and so on. So although the auto-correct works well for normal typing (in Mail and Notes), it's really only in Safari that you need the horizontal keyboard for inputting non-dictionary words.
Like a lot of posters here, I've got used to the vertical keyboard now and would rather have the extra screen space to see what I've typed. The only time I really struggle with the auto-correct is for passwords and that sort of thing.