My thoughts exactly until i played legendary. Trust me, just try it. Use the suppressor till their shileds pop then switch to a precision weapon.
Even on legendary i was able to take knights out with the suppressor alone.
Like most Halo games, the campaign usually makes all parts of the sandbox useful. Multiplayer narrows down the options considerably (but I think that's to be expected and necessary.)
Whoever is making the next |OT| just don't be a dick about it please.