While the low points in the menstrual cycle are close to males, females experience a significant monthly dose of progesterone. If I don't have progesterone in my HRT regiment I am missing out on something cis females have. Yes, it's more even, but we also have more even doses of estradiol.
For me personally progesterone helped even out my mood. I can't complain about the breast growth either. In terms of levels, I am higher than the low points cis females experience in the cycle, but I am well below the mean average levels in the monthly cycle. Part of the goal with the early days of HRT is to kick off puberty, part II. And it I would want to closely match that, as much is safely and technologically possible. I feel like leaving progesterone out would be missing the goal.