Your music is only going to be changed on your main computer if you opt to replace the original with the matched version.
I suggest that you read this https://discussions.apple.com/docs/DOC-4112.
...because that means if my laptop crashes or my ipod gets destroyed, all of my backups are going to be switched to clean songs, something I'm most definitely not cool with.
Not if you are backing up the originals to an external HDD it doesn't. iTunes Match is not a backup service and should not be relied on as such.
I don't understand why an issue like this has lasted for over 2 years. I'll just cancel my service and move on. I realize it isn't a back up utilty per se, but that was my main reason for purchasing it, that and to upgrade my song quality, but if it's going to give me a completely different version of a song I have no use for this product.
A number of mismatches will always result from a matching algorithm such as that used by iTunes Match.
Hoping that such a service would provide a faithful backup is understandable but can never be realised.
Certainly the service can provide a valuable upgrade for many Songs as a supplement to your own secure backup of the originals.
It also provide an excellent synchronisation and access for all your Songs across up to 10 devices.
The latter point is where much of the true value of the service can be found.
It is not ideal for all and there is certainly no shame in opting out if it does not meet your needs.
Sadly the promotion of the service has not always provided a realistic expectation of its true performance.