The main advantage I would see with doing a summer school at a school you plan to audition at is that you would get familiarity with the buildings, studios, layout which can help tremendously with confidence when it comes to audition day. At least that is how it seemed for my dd. Central offers shorter Easter courses which may give this benefit without the outlay of a full summer course.
For students who are already in the 6th form I think it is very wise to look for summer schools associated with companies. My dd did Northern Ballet summer school for many years and would thoroughly recommend it. The links with the company are close and The Artistic Director of the company comes in to watch the students. If we were looking now I would probably suggest she looks as the Scottish Ballet summer school and Ballet Cymru too.
But most important is that they get a good standard of training, a good summer school can really bring a student on. And rest during the summer is important too. Bodies and minds need time off to recover and rebuild and this reduces the risk of injury. Most dance companies give their dancers a good break during the year for this reason.