[IndexedDB] Any particular reason built-in properties are not indexable?

The spec today requires that properties key paths point at need to be enumerated (see 3.1.2 "Object Store"). Any particular reason for that? It would be reasonable to allow an index on say the "length" property of a string. Perhaps we're opening the door for too much, so I wanted to double check so we make an explicit call one way or the other. Thoughts?

Thanks
-pablo