I think that's fair enough. S-ranks are awarded for playing well. Simply fucking around will not be considered "playing well". However, if the Ground Zeroes ranking system is anything to go by, you have more leeway than in any other game in the series to get good ranks. You can kill a few people or get a few alerts, granted you do something else a bit better (you could finish the mission very quickly or you could rescue prisoners to make up for the lost points) so you can still use all the funny lethal weapons when going for good ranks.
Furthermore, the missions are replayable and should be fairly short so nothing's preventing you from playing through them several times to test out different approaches. You can just fuck around during one playthrough and then play the mission seriously while avoiding killing anyone. I'm glad the game offers these different approaches.
Well, not to mention this time around there's also the Free Roam option available sooo you won't have to worry about mission ranks while you're in that mode unless you explicitly choose to enter a mission. Then again, maybe I'm assuming too much about the free roam
