Oxbridge Application Watch
Cambridge consider "holistically", does this mean a weaker interview could be made up for in other areas.
My impression is that part of the "concept" of the interviews at both is to simulate the tutorial environment and see how you respond to learning in such an environment - doing poorly in that would probably signal the tutorial format at those universities would not be a good fit for you (and thus you might not do well on the course if they did accept you).
nexttime may have some insight...?
Cambridge both interview a higher proportion - off the top of my head it's about 3.5 candidates per place versus Oxford's 2.6 - and do fewer interviews per candidate ( Oxford normally do 4, sometimes up to 6) so it makes sense that it carries less weight there. Again though I'd be surprised if it wasn't the most important factor though.