Here is my take.
Yes sometimes they do look essentially the same. You unlock, your clock is in the same spot, your notifications are in a pull down. The graphics are different in most cases which can be a plus. maybe instead of a switch to turn on and off the autorotate, the brightness button is there. it isnt so much how it looks for me it is how it acts.
Even though each phone is built the same, they do not always act the same. I know from my Fascinate days that some UV kernels would save a ton of battery life on others phones, but every UV kernel would bring my phone to its knees. I try every custom ROM that comes out and leave it on the phone for at least a week (unless it makes the phone unusable) to see how it runs on my phone. I would not put on a ROM that had essential things broken (dialer, wifi, camera, you get the idea). What is essential to you may vary. Sometimes you find a ROM that works great but doesnt look that great to you, then you theme it.
It is all about variety and what works best for you and your phone. I appreciate all the ROMs that come out and all the developers that do all this crap for free.