I'm running JRMC18 on Windows 7 which is a dedicated HTPC / media server.
If it's working great now - there are absolutely no pros (no must have features or benefits to MC) in switching the entire OS out now.
Win 7 is mature, tried, tested and true while Win 8 is...not. And don't forget to factor in the hassle of prepping your old machine for a new install, backing everything up, finding drivers, troubleshooting the install after a ton of issues crop up...and then of course the Win 8 "experience"....I will leave that you you to consider....
For a dedicated HTPC that sits in the corner and sees almost zero user interaction with the actual OS - my money would be on most reliable and stable option possible.
And I am right in the trenches with you - I am about to build my very first HTPC in the coming weeks and there is no way I would ever consider using Windows 8 for it. It will be new challenge trying to "sell" a computer to the family for movie night as it is - never mind having to play "MS Tech Support" if the OS starts barfing half way thru a flick
If I was you and your machine is rockin as is - I could not find a single reason to upset that balance.
VP