Condition Coin Slabs

It is justified to have a coin slabbed if it has a value of over $100, even if it has a very low grade.
The advantage that a slabbed coin has is it's third parry grading, and so a a buy / sell price can be more easily agreed on via ebay, where the seller and buyer may never meet.

There is a tendency for British and European collectors to ignore coins below MS-60, unless they are very scarce or rare.

Americans are very sensitive about any high condition coin, even if common, in the range of MS-60 to MS-70, and so very often prefer Third Party Graded slabbed coins. The price range, even for common coins, in the range MS-60 to MS-70, can be huge, and thus Third Party Graded slabbed coins are often preferred.

There are still the 'old school' collectors around that simply collect any coin preferably unslabbed, provided the buying price is right, and is the best strategy for ancient and hammered coins.