Ok, I'll bite. Please provide specifics. Which private bank is offering this?
Do you work for a bank? A company doing marketing for a bank?
After a little bit of research I think I understand what this is all about. 90% of the Google hits are for the poster here. But it seems to be something offered to high net worth individuals with exposure to currency risk in other countries. For example, a resident of the UK with a mortgage on a property payable in euros or possibly in Norwegian crowns could make investments that pay dividends in those currencies to offset the risk of his own currency depreciating.
But I would say that for 99.99% of people, including most high net worth people, the fees far exceed any possible savings. But without a prospectus, it's hard to tell. Always keep in mind that one major attraction of selling things to high net worth clients is that the bank does not have to provide as much disclosure and can often rip clients off with fees that would be illegal if charged to regular people.
So basically, an ECU multi currency mortgage reduction facility is a ripoff in my opinion but I would be willing to change my mind after reading a proper prospectus.