The complications involved in getting the copyright permission is the problem, Howard. And often times the cost if it goes through one of the gatekeeper agencies. What you see as "doubtful" really isn't worth the risk. Just going ahead and then being confronted later by one of the agencies can wipe out a project. The same goes with publishing photos online - making sure they aren't held by Getty, etc., who are insidious in their copyright claims. (This aspect applies to disk cover art.)