Well that´s what multithread is all about, help the main thread (that are processing in sequencial mode) to process data faster, parallely.
Each Thread must be independent, using just syncronize methods to contact each other in the process. Most today bottleneck scenarios are, in fact, created by Software limitations, not Hardware. That´s why Mantle put AMD chips on par with Intel in games that support it, and i am aware of the Amdahl's Law.
i7 4770K = 8T CPU - M.T Sharing Resources.
FX 8350 = 8T CPU - M.T Sharing Resources.
Same scenario but the i7 have better single thread. In the end, just marginally diference in fps between both CPU´s, and both not 100% loaded.
Why such small diference since i7 S.T is way faster? Because M.T remove potencial single thread bottlenecks from the AMD system, that also exists in the Intel System, but less evident.
We must realize that even if the game engine uses 8T, do not mean it will saturate all of then. Each Thread Workload has its demands and purpose.
The way you say, we will never be able to see a 100% saturaded AMD/Intel 8T CPU, i do not agree. But we do know that we will not have 100% increase in performance per core count, with multi threading.
Nice "Amdahl's Law" explanation in the VID.
EDIT: For the 20fps Crysis1 dude, here a simple image of GPU load using Crysis1 maxed out 8X A.A Ultra
, GPU always full load, only one core being really taxed, CPU @ 5.0Ghz pushing R9 280X O.C (1100/1500) to its limits in a Single Thread game, and using CPU bound (low end) 1680x1050 resolution. No CPU throttling.Edited by jclafi - 2/8/16 at 4:02pm