I do think that one of the great advantages of an Oxford interview - relative to say Durham or Cambridge - certainly in my DD's subject, which is English - is that yes it may take a couple of nights' stay, but in the end, not only do you have a sense that they have genuinely chosen you - but also that your offer is realisable - AAA - hooray! (although it didn't always feel like that last summer, as others will remember!)
Not quite the 2Es of my day, but contrast Cambridge, where for English, even after all the interviews, tests etc, your offer is still likely to be a stress-inducing AAA - while at Durham, you must achieve at least one A*, although bizarrely, they don't actually care in which subject you manage this, which I personally don't find very confidence-inducing, in terms of thinking about the skills of the individual candidate.
The other thing about Oxford is that as I understand it, they don't significantly "over-offer", because they assume that if you are prepared to put in all that front-loaded effort, you must want to accept their offer. This means that you (sort of) have the comfort of knowing that if you, say, miss a grade in an unrelated subject (which was my DD's big worry!), you can have a conversation about it with somebody who has actually met you, looked at your work, and has decided they would like to teach you. This, IMHO, makes the three day visit "worth it".