Semi key dates are hard to get coins in a series, but aren't the rarest in their series.
For example, with Mercury dimes, the 1916-D is the key date, they only made 264,000 of them, even well worn 1916-D Mercury dimes sell for hundreds of dollars.
On the other hand, the 1921 Mercury dime is considered to be one of the semi-key dates, they made about 1.2 million of them and in average condition you can pick one up for about $30 or so.
Semi-key dates sell for more than a common date coin. For example, a Mercury dime that is in average circulated condition sells for about $3 or so. However, a semi-key date like the 1921 would never sell for that low (if the dealer knows about them). So while semi-key dates aren't going to make you rich if you find one, they are worth quite a bit more than just an average coin.