Letterboxed films converted to Anamorphic may degrade in quality if done "on the fly" by just stretching to fit your TV screen and such.
When done appropriately on a PC during encoding, the image can look better than the original.
There are many resize methods available that are able to find edges or nearest neighbor pixels and intelligently resize to provide more image not just stupid line-doubling.
I have converted to anamorphic in a few projects and the final quality is almost always superior even if it's not "true" increased resolution.
It's why there is a monster-size thread discussing how to convert GOUT to anamorphic for personal home use.
It's why many SW preservations have gone anamorphic or better (Citizen's wonderful HD Xvid anyone?).
So yes, anamorphizing a laserdisc may effect clarity... it would be improved.