MacUserAE --- based on my experience over the last few days, I would urge you to STAY AWAY from iTunes Match until Apple can better support the concept of multiple Apple ID's sharing content. After purchasing the Match upgrade about a week ago, I've spent perhaps 20 hours of my home life dealing with Match-related issues. What you are fearful of - that you will somehow be locked out of your ID and your purchased content - is exactly what happened to me.
I also thought that iTunes Match would be a good way to consolidate and then share music/video/apps between family members. I purchased the service with 2 goals in mind: share content in my house via a Mac Mini server, and also "consolidate" the content my wife and I had purchased across three separate Apple IDs. I am not a technology novice. I read the various online boards, read the Apple support pages, and based on what I read was convinced that Apple's product would deliver exactly the benefits I was looking for.
I was wrong.
Apparently I did not migrate or associate my hardware (2 notebooks & a mini home server) in an Apple-approved sequence. After putting about 10 hours into the project, I discovered that I had "used up" my computer's associations and could no longer download previously purchased items. I am now missing 1000's of purchased iTunes songs and videos. I am not permitted to download or recover them until the 90-day "timer" runs out on each of my computers. (Currently 84 days & counting). Ironically, I have an exceptionally "clean" and easy music collection: I have literally no shared/ripped/illegal content, as all of my media files were either uploaded from a purchased CD or purchased through iTunes.
When I contacted Apple for support, I encountered a steady stream of suspicion and poor/misleading information from the different support reps I spoke to, each of whom seem to working independently. Support is apparently by Chat only, and much of the dialog seems canned, or pre-written. For eample, the last support rep I dealt with first described his own experience setting up match using these pre-written cut & paste "snippets" that had obviously been prepared earlier, as they did not actually address the questions I was asking. Later in our encounter, he seemed to forget what he had already "pasted" and he wrote (in real-time) that he had actually not had any first-hand experience with the product.
Discouraging all around.