The hardcore coin collectors call the slabs "coffins." There are numerous reasons why they dislike sealed slabs.
You can find threads on CoinTalk about how best to "rescue" a coin from a slab without damaging it.
I have to say, even though I'm not a hardcore collector, I can understand the sentiment. I mainly bought slabbed coins just for the supposed "protection" of being professionally sealed in an air tight container, only to discover that it wasn't so air tight after all, and some of them are starting to tarnish. In a slab, they don't "tone" evenly. Of course a 24K gold coin doesn't tone, so it doesn't matter what you keep it in as long as you don't scratch it. But 22K gold coins (circulated gold coins) can tone somewhat over time.
I really prefer coins that I can hold. Something like a Vicky Crown or Sovereign that was circulated (Queen Victoria "Jubilee Head") You can often find these on EBay close to melt prices, and every once in a while you'll find one below melt. The sovereigns were and still are minted as bullion coins in large numbers. If you will recall, in one of the 007 movies, Bond had a briefcase with sovereigns hidden in the lining.
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At one time I was collecting pieces of eight. All of them are well circulated, so they are safe to handle as long as you wipe them down afterwards.
I think my favorite bullion coin is the Isle of Man Angel. I have a 1/10 oz and a 1/4 oz. I seem to have a hard time finding them.
They can be bought at close to melt...if you can find them