Nothing new about this many fiats suffer with premature disc failier its the cheap and cheerful parts that fiat fit to their cars..my advise would be dont use them again and go for mintex or ebc and they will last longer.. welcome to the forum as well..
Who says they need replacing? It might be a dodgy garage after your money!
How many miles has your Multipla done? What may have happend is that the original brake pads were worn down to the metal, scored the discs and then were simply replaced. This might explain why the pads are 30% worn and you've been advised to change the discs.
It's not a major job and shouldn't be that expensive. Alternatively, do it yourself and save a lot of money