why is uneven roasting bad? if you want complexity in the coffee wouldn't a variety of roasts give variety / depth of flavours? is there some technical reason while all beans should be roasted the same?
I've never really thought about this per se, but the first thing that comes to mind is that if things are uneven, it's generally more like some beans are burned and/or some aren't roasted enough. Either of those scenarios are going to produce undesirable flavors. I think the more common way to capitalize on a variety and depth like you mention is to roast different types of beans separately and then mix them. That said, there's a lot of complexity and variety just in single bean (or, single origin) roasts, differing levels of roasting bring out different flavors.
Would you like half of your steak well done and the other half rare?
Steak is normally not ground, unlike roasted coffee beans. So there is no mixing. On the other hand, I know at least one coffee company that sells a mix of dark and light ground coffee (Melitta, in the US).
If you were to blend the entire steak together, would you notice?
Yes, it would taste like a mix of rare steak and ruined steak
I think it would. It's not like a piece of charred meat 'cancels out' a piece of cold, uncooked meat. If you were flavoring a batch of yoghurt, and 1/4 of it has 10% too much, and 1/4 10% too little, then yes. But for processes where the not-measured characteristics of the elements on the left side of the curve are different than those on the right, no.