Because what we are actually talking about here is not the importance of scenery, or even the value of uniqueness in trees - we are discussing the specific level of uniqueness present in the trees...
*Where's that forest gone? ... there are trees everywhere. How are we supposed to find a forest in the middle of all these trees?! lol
I have no idea what to rank it, as I don't got a list of all important features in Skyrim in my pocket
All I know is that it's important to have a tree variety. It's a huge part of what landscapes consist of, obviously...
I hope to see more of the game to prove myself wrong, but so far it looks like not that much variety. It could/should certainly be much much more, look at Vurt's Solstheim mod. And it does look great to have a variety, like with pretty much anything else.