Yes very good question. Have to say when we did roll out esx5 across our environment we asked ourselves the same question.
Our branch office hosts were standalone and thats really were we asked ourselves was it worth enabling (our DC's run clusters so was
From delving into the various articles it became clear enough for us that it was very specific on it being a "datastore-level".
From reading in particular:
"Without setting the shares you can simply enable Storage I/O controls on each datastore. This will prevent any one VM from monopolizing the datatore by leveling out all requests for I/O that the datastore receives."
As a result we DID enable on our branch offices. In particular paid attention to our branch file servers. We found once contention DID hit that indeed storage I/O kicked in.