The sentinel beam's usefulness is limited when you would use it. Probably the best use for the Beam is Flood infection forms.
All you have to do is tap the trigger to kill the infection form, and when the infection form is in the reticule, the beam will auto aim. This can be helpful for taking out many infection forms without using much ammo. It also works well against other types of flood, but one thing to note is the beam overheats very quickly. You will have time to kill one, maybe two combat forms before the beam overheats.
This is also the case against stronger foes like elites or shielded sentinels. These are not enemies against which you would use the sentinel beam.