If you were on a jury, and someone said "I'd had 4 pints so I can't be held responsible for taking her drunken enthusiasm at face value", as their defence, would you think that stands up as a defence, or would you dismiss it and say that after 4 pints, for that person on that day, they should still have been in a state to recognise that the other person was not in a state to consent.
The drunkenness scale is that of being tried by a jury of your peers - if your peers come to the conclusion that it is beyond reasonable doubt that (a) the other person was not in a state to give consent and (b) it is reasonable to think that you should have been aware that the other person was not in a state to given consent, then you'll get convicted. If they don't, you won't.
Would a jury convict two drunk teenagers who slept together? I doubt it. Would they convict a more-or-less sober teenager who slept with a very drunk teenager? Probably yes and so they should - predatory fucks who go out looking for wasted women are not exactly uncommon. Two people who get drunk, meet in a nightclub, shag and can't remember it will never ever be convicted under this law. We know that because Clayton McDonald was acquitted, due to the circumstances in which he met the woman in this case, whilst Ched Evans was convicted, due to the - wildly different - circumstances that he met her.