I think a lot of replies to this thread have been unduly critical.
It is perfectly understandable why somebody would be annoyed at a massive cut in their overdraft limit in the same way as they would be if a credit card provider suddenly reduced their credit card limit by a similar amount.
As to why somebody would want a (large) unused overdraft facility, the obvious reason which comes to mind is the security that, whatever happens, no direct debit will be unpaid or cheque bounced as a result of any unexpected occurrence. Also, a high overdraft facility never used would also give potential lenders peace of mind that their direct debits will be met.
I do agree that Â£10,000 is a bit high; I have always felt that a facility approximately equal to the net monthly income paid into the account was reasonable and appropriate.
I disagree with that bit, I would say lenders are likely to consider it unused credit and it will go against any applications you make for further credit. They will take into account the overdraft limit when deciding whether or not to accept a credit application and what limit they will offer.
No lender is going to be happy with someone paying off their credit card direct debit for example by borrowing from an overdraft. That signifies the start of a large build up of debt and makes it more likely the person will default in future.