Assuming by "discreet" (discrete) you mean the distinct placement of an instrument or a sound in a specific point in space, 5.1 can only achieve this in a horizontal plane - either directly in one of the 5 speakers, or by imaging between speakers. By adding Atmos height speakers, you now have the ability to place a distinct sound anywhere in an invisible dome by imaging using any combination of the speakers around you and/or above you. There are many examples of Atmos mixes that do this effectively, elevating a discrete mix into the 3-dimensional space around and above you.
It is very true that there are many 5.1 (and 4.0 for that matter) mixes that blow away many Atmos mixes, but that is due to the mixing choices made by the mixers and the fidelity of the material they are working with. For example, the 5.1 mix of Dire Straits - Brothers in Arms is far better than the corresponding Atmos mixes of the same tracks (different mixers / different choices). But all else being equal, I prefer Atmos over 5.1 (for example, when Steven Wilson does both a 5.1 mix and an Atmos mix of the same material).