I'm in the same boat as the other folks here. The ID I'm using now is the ID I got back when the iTunes Music Store first launched; I was buying music online pretty much immediately, using a Yahoo address that was, at the time, my primary e-mail address. Meanwhile, I have a second Apple ID that I got when I signed up for .Mac. That ID is My.Full.Name@mac.com. I held onto that through the MobileMe thing and have now converted it into my iCloud ID. I've been using it for a good four or five years now, it's the e-mail address pretty much everyone has for me, and it's tied to all of my bank accounts and so on. It's also the account that syncs to my address book, my calendars, et cetera, et cetera.
The problem, of course, is that the one thing my iCloud ID won't sync to is the one thing I was most excited about when iCloud rolled out- my iTunes purchases. I've bought a heck of a lot of stuff on the iTunes Music Store, but it's all been done on that old Apple ID because that was what was available when iTunes first started selling music. So now, based on this thread, my options amount to swapping out iCloud IDs on my phone depending on what I'm doing (a pain in the neck) or having to suck it up and deal with the fact that I'm rendered unable to use Match because I was silly enough to jump on board the iTunes bandwagon long before .Mac (let alone iCloud) was a twinkle in Steve Jobs' eye.
There are various other services I've used where it has been possible to merge accounts, purchases, et cetera. It seems really weird to me that Apple cannot/will not facilitate this option here, in a situation where it's arguably the most necessary. Why make this so needlessly complicated? And it's clear from this thread that I'm not the only one in this situation. If we have twenty-plus pages of people frustrated enough to post, how many people are out there that would be happy to shell out the money for Match but won't because they can't synch it to their iPhone or iPad (which is the whole point)? Probably a lot. As it is, I'll likely end up paying for something like Spotify instead.