That forces you to deallocate resources in the inverse order of allocation, which may or may not be what you want. For example, the lock acquisition and release pattern used in the implementation of Lehman-Yao trees (and variants thereof) is inexpressible using monadic regions.
Hmm, well I've never implemented this myself so I'm not going to contradict you, but I'd be really surprised if you can't hoist into the monad transformer stack and close off a lower region earlier than the top-level region.
I'm pretty sure DBMS implementors have to work with data structures whose nodes are spread across several files, and which have to be locked and unlocked in very precise ways to preserve the integrity of the data they contain. I'd like to be able these locking and unlocking patterns with types.
