Once the scratches are less than the working distance mentioned by RedGrittyBrick it should pose no physical problems to the laser or eye.
Reading a minimaly scratched disc is ok, the builtin error correction easily handles this. But reading a severly scratched disc can cause the laser to move excessively back and forth re-reading the area of the scratch. You can hear the movement of the eye as it moves back and forth when it attampts to read a severly scratched paort of the disc.
If the disc is really bad and after a few seconds to minues of re-reading, the drive will report an error for that sector(s). It is up to the application to report an error to the user or to continue to the next sector. For movie discs (in most players), the next sector is read and the image is partially reconstructed. That is why you may get some artifacts in your movie for a split second. Data discs on the other hand, will pose a problem. Similar to a bad sector on your HD that piece of data is "unrecoverable".
Now comes the tricky part. You can attempt to recover the bad sector by reading the area over and over. How can this work? Unlike a bad sector on your HD, a scratch is a surface defect. The data is most likely intact on the recorded surface. Reading over with the laser can get by some of the scratches to the data. Once enough data us recovered, error recovery does the rest.
And finally this excessive reading over and over will wear down the drive motors causing the drive to malfuntion. It may not burn as well or read data as well as before.
All of this is from my experience of performing data recovery on optical media. The drives do fail much faster than simply burning or reading good discs. Interesting enough a firmware update to the drive can prolong its life until it starts to malfunction again.
My recommendations, if you are just burning/reading discs your drive should last a while. For data recovery, expect to repalce your drive much earlier. Store and handle your discs properly to prevent scratches.
Other advice, when burning, make sure to verify. Optical media do decay over time so I suggest you verify your data after a few years. Use error recovery generation software (quickpar) to generate recovery data to in the event you do meet an error, it can be recovered.
Hope this helps,