I get what you're saying to some extent, but in my Y13 classes this year I've had students interview for:
Oxford,
Medicine
Dentistry
Physiotherapy
Paramedicine
Nursing
Primary Education
I've also had students taking external admissions test this year, as well.
All these students had interviews on different time scales, and I have some students who still have upcoming interviews or haven't heard back from all their choices. Whenever mocks are scheduled, it will impact someone!
When we do "official" mocks, we deliberately use a paper that will be unseen for the students and different to last year's mock paper. People sit the mocks on the day or not at all like the real exams. Part of it is about preparation for that environment, but also to try and give everyone a realistic idea of how they are performing at the moment.
Personally I do think mocks between Decemeber-Feb of Y13 are pretty important, as they help students gauge a likely outcome for the summer, and hopefully make informed choices about their uni offers.
We don't often have students get in to Oxford or Cambridge, but when we do, they are genuinely exceptional, and usually comfortably getting As. Having "a lot on" might impact their performance, but that likely means scraping an A rather than being a good 10-20% inside the grade boundary. We do try to offer support in terms of mock interviews and guidance through the process, but we can't change things that impact other students to suit someone applying to somewhere prestigious.