The primary reason is to not be left at the mercy of the vendor. With vendors increasingly struggling to maintain their revenue streams through forced upgrades, they are relying increasingly on timelocking their software licenses. Thus, you have to re-license the SW after a period of time determined by the vendor, and your valuable info may be unavailable until you do that. Open formats are a form of insurance against that happenning, especially if they happen to be also supported by one or more open source products.