My ideal/hope would be that both Couchbase and CouchDB move to FoundationDB and retain maximum compatibility. It makes future portability decisions a lot easier (e.g. I can start with Couchbase now and switch later, and/or have CouchDB and Couchbase servers co-existing using the same FDB deployment).
Another (similar) reason, is that with the FoundationDB layers concept, multiple layers (and/or raw KV usage) can co-exist in the same deployment, which makes operations a lot easier; for example, if I store Couchbase databases alongside a custom hashmap or other bespoke structure using FDB KVs, then they are automatically stored/backed-up together.
Finally, I strongly believe in the (design, software, testing) quality of FoundationDB. Here is a classic quote from Aphyr (of Jepsen test fame):
So there is an appeal to using/trusting FDB for the underlying distributed database, and building (more expressive/simpler/stateless) layers on top of it.