I agree with Erik that center channel is probably the most important speaker in a HT system. In movie audio production, the center channel is where the source of vocals are generated from. However, the left/right channel also has a tiny bit of vocals. When produced, the movie soundtrack uses left/right channels for ambience. The vocals are usually at about 10% and have a slight delay. This gives you the reality of sound where the actor is speaking from, such as talking in a bathroom, talking outside next to a building, etc. Without the left/right vocal ambience, the sound may be very mono.
I have listened to movies/shows using only 2 channel and the processor has to compromise by sending the center channel to both left and right, while at the same time playing normal left/right soundtracks. What happens is that the vocals and sound have an "echo" effect. It makes it harder to hear voices.
Left/right surround speakers do add ambience. It makes the movie experience more immersive or "stereo like". If you only have left/center/right, the sound is more "mono" in a sense because it is all coming from ahead of you. The surround speakers put you more inside the movie experience.
I have been in full Atmos theaters with ceiling speaks and tons of surrounds. The effect is interesting with sounds coming all over the place, but honestly, I would rather have an extremely great 5.1 system than an average 11.1 Atmos system. For me 5.1 is enough to really enjoy movies, if the system is high-end enough.