A more competitive product or a slimmed down product mix?

GM is considering cutting Pontiac, Saturn and Saab in order to save money? Begs the question – when companies merge what benefit is there in having more than one vehicle that are similar within the same company?

Example:

Chevrolet Traverse / Satrun Outlook / GMC Acadia / Buick Enclave – all four are crossovers and have virtually the same features are built on the same chassis – so why make four versions of the same thing? Why not just one model with all the different levels of features?