It will vary subject to subject @goodbyestranger, but none of DS' weekly essays went towards his yearly grade.
I'm not saying the work isn't a lot, but an essay is an essay. They can put as much or as little into it as they see fit, just like at any other uni. In general, life is easier at Cambridge than many other unis where they might be in house shares all over the place and dealing with inconvenient commutes, shopping, landlords etc. Cambridge is very walkable / cycle friendly - you can get anywhere and it's easy to find people. It's not like negotiating Manchester, London or Bristol, not is it a remote campus Uni like Warwick. This leaves more time for rowing, choirs, CU, etc.
Also, I didn't mean to imply that tutors are clueless. Not at all! What I meant was, if you have 50 applicants, all with top grades, but can only choose 7 of them, what's to say that the rejected students wouldn't have fared just as well at Cambridge as the ones selected. Probably most of them would have done just as well. Apart from a very few possible genius-types, I don't think a student with three A stars at Cambridge is any different to one with the same grades elsewhere.
Yes, I do think Cambridge could do a lot more to standardise their admissions process. How can it be fair that some colleges ask for two essays, while some ask for no essays for the same subject? Some colleges may do one interview, others two - again for the same subject.
If they are 'pooled' from a college that required no essays, would this mean they are less likely to be picked up by a college that does require essays? Who knows? It's very opaque. I think they could at least standardise the entrance process, per subject, across all colleges. I think the Oxford process sounds more standardised.