wouldnt be bad for fiat, but i cant see an overall benefit for BMW. cost savings may be smaller than sales losses.
many/most mini drivers want to buy a slice of the BMW brand, even if they dont admit to it. if the next mini was built on a platform that was also used by & developed with fiat, it would no doubt deminish the BMW effect that the mini enjoys. if that same mini also had a fiat engine it would have a huge effect. unlike most platform & engine sharing that goes unnoticed, this mini example is the kind of situation that the press would publicise.
for example few people know the corsa shares it platform with the GP, but everyone knows the leon is built on a golf platform. that is because every leon review makes a point about the golf underpinnings, but corsa reviews dont usually mention the GP relation.
check out the corsa parkers review for example-
http://www.parkers.co.uk/cars/reviews/vauxhall/corsa-2006.aspx
no mention of the GP anywhere
then check out the parkers leon review-
http://www.parkers.co.uk/cars/reviews/seat/leon-hatchback-2005.aspx
the very first thing is says is the golf issue!
only big news gets coverage. a mini with fiat platform and engine would be big news because the mini uses its executive BMW heritage to justify the price and prove the quality. you cant use a brand built on BMW for a car built on fiat. it would be a lie, and that would be big news. even your mother would hear about it.