Top scale grades should be difficult to acquire, even in those cases where you are getting it as direct as possible with minimal distribution and handling into a clear plastic crypt. There is always a chance for some imperfection. It's meaningless really. You can have a game that has transfered hands 5 times come out of grading better than a fresh from the case copy. All it takes is a little carelessness with a case knife.
The only value I could possibly get out of buying a slabbed game is the third party verification that it is what it is claimed to be. Fortunately, most the people I have dealt with have been honest in representing the items they sell and there is always a means to correct in the few times it has gone bad. So, third party verification isn't worth the expense. Maybe if something being bought was a unique item and very expensive, then the extra cost is negligible. Like say a sealed Stadium Events... I can understand slabbing that sucker.