There are many reasons to share routes between routing protocols, including the following.
- Migration from one routing protocol to another requires a temporary period where both legacy and new routing protocols are active on a network at the same time
- Applications require different routing protocols for optimal efficiency (perhaps there are UNIX host-based routers which require RIP support)
- There are political boundaries which result in use of multiple routing protocols (and perhaps a department doesn’t want to upgrade a router to support a new routing protocol, etc)
- The network runs a mix of vendor technologies, where some run EIGRP and other devices to not support it
Stuck-in-Active: Journal of an IT-Network Administrator 