They are not a basic element to the design of the amp, Pete didn't have them implemented in his original design. They were added in when dsavitsk (I believe) added in his improvements, and can be left out. I know for a fact that Beftus had at least one SSMH where he disconnected C7/C8, he noted that it reduced the overall gain but otherwise didn't change too much.
I stand corrected on the caps being part of the original design. Must be the starving part that reduced the parts count.
That being said, those caps are still a good idea. There is a signal being formed across the cathode bias resistors that is in phase with the input signal to the grid. This is a form of negative feedback that the capacitors eliminate. This is also why the gain goes down when the caps are eliminated (introduction of negative feedback reduces overall gain).
Perhaps it might be worth experimenting to see which sound is preferrable.