Making the same vehicle for 2 to 3 different brands isn't the problem, its making them and overlapping them pricewise where they compete with one another is the problem.

Example: Chevy Impala LS, Buick Regal LS, and Pontiac Grand Prix GT. All same platform, same 200hp 3.8L OHV V6, and same $25k msrp starting price. Sure each one is a different "brand" with cosmetic and ride differnce but its not that hard to imagine them competing with one another since most consumer starts out buying their cars based on price.

Compare this to say the Camry and ES300 and see how the ES300 is a step up from the Camry quality and price-wise and how they do not compete with each other.