Interesting that this has come up.. It was mentioned on another site that had forum icons asking if they were really necessary and in that topic someone tweaked the code and made it so you could remove the icons and made it look the way xF does it currently.
I like the lack of forum icons and the way it's done here.
The only thing is that we used the thread icons to put our news items in categories which vbadvanced then displayed next to the news item. I think we could use prefixes to do the same thing, display a prefix icon next to each news story but might take a little figuring out once a portal/cms is developed.
I used post icons all the time specifically for certain areas but that is my preference. That said, I was an advocate for post icons to be introduce in the first month of xenforo but feel post icons would be redundant and bloat xenforo. With that also, 99.9% of people would never use post icons so I feel not introducing them in xenforo was, and still is a good move.