It's certainly worrying that you are getting so many like this. As icey stated above, it usually means faulty media.
A long time ago before I knew any better I had some Laser branded (SKC mid code) discs that did this and every one of them was a reject.
More recently however I got a Ritek G05 disk with a big ugly streak in the dye, almost the whole width of the disk. Thought it didn't have a snowflakes chance in hell of burning properly, but surprisingy it burnt ok and even PI/PO scans ok.
The bottom line is that any patchy discoloration is a bad sign, but if the scan says its ok then it probably really is ok. I'd still worry about having too many like this though.