I mean, it's only 70 Power, and yes, it has a 50% chance to lower the foe's sp. defence, but that's still not very good. Giga Drain would be a better attack. It does more damage and actually heals your Pokemon, and it has 10 PP. Why does Mist Ball and Luster Purge only have 5 PP?
It's probably because Luster Purge and Mist Ball are the signature moves of Latios and Latias.
Latios and Latias are also legendary Pokemon, making them seem very powerful, if a move has low PP and low accuracy, it's either powerful or trying to seem powerful. Most legendary Pokemon signature moves have low PP, (Five-Ten from what I've found) unlike some signature moves of Pokemon that aren't legendary.
Hope this helps!
They're signature moves
Only Latias can learn Mist Ball, and only Latios can learn Luster Purge. Thusly, they have low PP. Also, I'm pretty sure they're the only moves to have a 50% chance of lowering an opponents Sp. Def., so that might play into it.