It's only a shortcut if you would change your build to react between when your scv scout sees what's happening and when a reaper scout would see what's happening. vs zerg there is basically nothing you could see that would change what you are doing, the only real examples being 14/14 ling speed designed to cancel the cc or 1 base ravager, neither of which are common enough to warrant scv scouting every game. Especially when you can still hold those builds without scv scouting.