Question about the use of NO_EXPORT in BGP route announcements

["Sriram, Kotikalapudi \(Fed\) via NANOG" <nanog () nanog org>]

For some IETF work in progress related to Source Address Validation (SAV), it is useful to know the purposes for which 
NO_EXPORT may be attached to routes announced in BGP, especially towards transit providers?

I know it makes sense for an AS to announce an aggregate less-specific prefix to transit providers and peers without 
NO_EXPORT while announcing some more-specific prefixes (subsumed under the aggregate) with NO_EXPORT towards customer 
ASes.  But are there good reasons when an AS might announce a prefix (route) to a transit provider with NO_EXPORT 
attached?  The IP address space in consideration here is meant to have global reachability.

Sriram