I've bought and installed both RoC and TFT way long ago and used to have the original TFT disc in my CD-ROM/DVD-ROM. It worked just fine (surprising, huh?), until WC3 poped me this dialog, roughly like this; "Please make sure you have the original WC3 TFT disc inserted". Since I had the original TFT dics inserted, I ejected it, picked it up and found some minor scratches. To be on the safe side, I decided to emulate the disc instead.
First, I tried to used ISOBuster to make an image of my TFT disc. That didn't work, so I tried to make an image of my RoC disc instead, since you can use both to run TFT. This time I got the same error from ISOBuster, but when I loaded the .cue file (with Securom emulation enabled) in Daemon Tools, it worked! Furthermore, I have never seen that dialog again.
I really don't know what Blizzard thinks about this, but I say it's OK, since it's just an image of my original, bought disc.