i know game freak wouldn't make them so hard to get if they were useless.
I don't think that they are really hard to get, but often they are expensive to buy. The reason is because they are very powerful moves, even though on average they only have 75 power. There are a few places you could use them:
* Most of the Pokemon with high defense are rock or steel types so this may not be a good strategy.
Giga Impact is good on Slaking.
Both are apparently "acceptable" with Choice items, especially in-game