I happen to know of two CIOs of very large enterprises who went with LDAP instead of AD with a very clearly stated goal of not being locked into a single vendor.
So it's happening, even if it's just a trickle at the moment.
But to me, comparing LDAP to AD is like comparing an engine to a car, isn't it?
LDAP is a protocol, it is not a standalone service. Don't you also need a client-side authentication mechanism and server-side sharing system that is seemlessly compatible with LDAP?
Insert below mandatory answer about how great existing open-source solutions are.