I subscribed to iTunes Match the other day (it just came to the UK) and was thinking - now that iTunes Match is backing up my library, ratings, playcounts and all that awesome stuff, I no longer need to include it in my time machine backup, right? It's matched 99% of the songs in my library and I'm not going to miss the others a great deal - I use Spotify very heavily as well, and if I don't need to backup my music (~100gb) I no longer need a larger backup hard drive :D
At this point in history, it seems unlikely that Apple is going anywhere, what with their gigantic cash reserve, profits, market capitalization, etc. That said, many apparently successful companies have collapsed and closed quickly, and the data they hosted sometimes disappeared. Less dramatically, individual accounts can get deleted, passwords lost, or access revoked for one reason or another.
In a nutshell, iTunes Match should serve as a backup. How much peace of mind would it gain you to have your own backup of your music in the unlikely but not impossible event that it fails you?
I have a rule which I apply to backups: Use more than one approach. I have an iPod Classic with an 80 GB HD, this is used as an external HD to store my entire iTunes Media folder and is backed up weekly using Carbon Copy Cloner.
I back up everything on my Leopard & Lion Macs with Time Machine and alternate between two different external HDs, on of which is kept off-site at a location I visit each at least twice per week. This HD is only at home for 36 hours at the most.
I also use two Carbon Copy Cloner external HDs so I have a bootable clone should it be necessary.
I have copies of my Web Receipts folders stored on Dropbox so I can refer to receipts from one Mac while using another.