I noticed that too, it'a an interesting little tid bit and not necessarily an issue at all so long as you keep subscribing.
As I said, In my case all music was previously downloaded and sits on my Mac Mini. I always tried to keep all music downloaded to that machine but previously I wouldn't have bothered backing this up since I could just re-download it and all... but if my versions are DRM free (since they were all downloaded under iTunes Match) it might just be worth making a copy of them all now before I forget and lose them to the machine.
Most of it was ripped from CD's long long ago, but after running iTunes Match over it all I deleted it all and downloaded the shiny new versions from iTunes complete with cover art and all the correct and complete meta data (I had keyed all mine by hand or used the auto stuff provided when ripping it along with scanned album covers). The physical CD's are either in a box in the shed, or far more likely, tossed in one of the moves over the years.
I've been 100% digital since the late 90's... moreso after getting a Siemens SL45 that could play MP3's in '01 and various other MP3 players after that.
There is plenty of stuff that wasn't matched for one reason or another, and the vast majority of I probably wouldn't even notice... But it's a little sad to think it's now locked to Apple for all eternity if I don't keep a copy of all those non DRM'd tracks.