Having been on E alone for 7+ years or so before ever starting P, the old perspective I'd adopted from my hrt-specialized physician that I don't need P because it's "not necessary for feminization" just seems ineffably skewed.
I'd been suffering for years with all the effects of estrogen dominance syndrome. I started P and within a few days everything just started opening up - I eat more (good for breast growth... my first question when people complain about hormones not working: "are you eating sufficiently to promote growth?") sleep soundly, and am just less stressed out.
Physical effects are great, but I can't see why they should warrant any attention before first checking the conscious and sub-conscious psychological effects of one's hormone regiment. In fact for some like me it can be hormonal imbalance effects making one think it's the hormones not working, or that one is just not fully responsive to feminization, etc. Essentially, P at this point seems to have a greater hand in my wellness than even estrogen, and there may well be potential hormonal / psychological imbalances simply inherent to not including progesterone. Whoever described the feeling with P as "balanced" put it succinctly.