The motor would be gettng 8.4 volts until the capacity drop dropped the voltage down. Or you did something to limit current.
Im gonna say that since 7.4 volts is only nominal it can cause cause quite a bit of confusion on "will this work? It only says 7.4, but a battery is 8.4...etc.)
I think they should label it as a 8.4 volt battery, as technically, thats what a 2s really is.
(Im talking about 2s by the way)