Many people only use products with a heimishe/chassidishe hashgocho, mainly because they want the additional stringencies that may exist with these hashgochos. When such a consumer sees a package of Mike and Ike with just an OU, s/he wont know if the product was manufactured with the standards that s/he wants. The heimishe/chassidishe hashgocho is just saying that this product conforms to those standards.
Assuming they piggyback off the original hashgocho, the consumer should be able to consume the same product even without the additional hashgocho sticker on it.