Well it is Microsoft them self who declare this http://blogs.msdn.com/b/ie/archive/2012/06/06/moving-the-stable-web-forward-in-ie10-release-preview.aspx and Mozilla. Is official information not my ideas. So “No real practical reasons” is exactly why not to include, not the other way. For as much as it matters it is you who should argue why they are there 🙂
Depends who you ask if you get pad on back or not. You CAN argue not used code should stay if you find the right people. Most sane will go “stupid not to, less is better, about time, weeee” while others like CSS Lint will be the last to remove. Also validators/generators will need to be updated, as impressiveweb hints. Zzzzz. Check ColorZilla for example, nothing happens. Well CSS Lint also have concerns about IE6 usage and do test for it! In fact if you check what they test for it is pretty much impossible to please everyone at the same time. Frustrating/annoying so if there is a chance to simplify CSS, regardless of how little, I vote to take it. Not the same as micro optimization.
But it can be a bit tricky to lean on “authorities”. Like CSS3Please. One of the go to sites for this. Lots of credentials etc. Yep, and they also recently removed “support” for IE8, so all those people who have been told just go there and get your stuff fixed, are now dismissing IE8 with certain things. That is opinionated and I think wrong. So best to be conservative and follow official announcements from vendors – which is why I posted.