If MC is included in standard 8 (which isn't totally obvious), and as a chargable add-on in 8Pro, then I would suspect we'll see little/no changes in MC in 8. If it's not included in 8, and there's no way of adding it, so it's only available as an 'economical' add-on in 8Pro, then we might actually get something that is worth upgrading to. It just really depends how MS are going to price these different editions.
I'd pay for 8Pro AND the MC add-on IF there are sufficient reasons to do so. If MC is the same as in 7, then I'll stay with 7MC. I just won't make sense to pay for these upgrades/add-ons if we won't see any difference to 7MC for dedicated HTPC's whose front-end is permanently set to MC.