Very true samjimmy - I have always advocated doing exactly that. But - you had better be able to properly and accurately grade coins yourself or you might end up losing a lot of money. It is not something the inexperienced collector should do.
I guess I do more comparing than actually being able to properly and accurately grade. When I find a coin and grade I like, I try to find the same condition coin in a non-top three holder and it seems to always be less. That works for me (a small volume collector). It wouldn't work for people who buy larger volumes.
And that, in a nutshell, is the exact problem. Part of the reason for the TPG system is that, to an extent, the inexperienced collector should be able to "buy the slab" with an expectation that the coin contained therein will at least approximate the grade posted. I will not accuse all of the lower-grade TPG's of criminal intent. I have no doubt that some are genuinely trying to be accurate. As a result, it is possible to buy, for instance, an NTC MS65, and get an MS65 coin. However, it's also possible to buy such a coin and get an AU, because of the inconsistency of the grading. I would maintain that finding a true AU in a PCGS MS64 slab is a vanishingly rare occurrance. I'm not railing against the alphabet TPG's here and elsewhere because of any injury I've suffered. I can grade the coin well enough to accept the blame for any mistakes I might make. Therefore, I'm someone who really doesn't need the slab. In order for our hobby to grow going forward, the people who *do* need the slabs need to be able to reasonably trust them until they can acquire their own grading skills. It's unreasonable for the system to require a newbie to only buy coins that cost $100 or less for the first 5 years, until they can reliably tell an MS63 from an MS66. Any company or concern which makes this process more difficult needs to be in our crosshairs, because to fail to act is to enable them, and to damage numismatics as a whole.