So, I don't really understand the second part of the question.
Stat-boosting moves are not always viable. For one, most of the time, your opponent won't just let you keep using them. So, moves like that need to be useful, and incredibly efficient. Denfense-boosting moves are.... not efficient. Most of the time, you're gonna want to boost an already substantial stat, and just boosting you defense means that the opponent can continue to attack you. Plus, since you can't heal your 'mons with potions in most competitive formats, unless you've taken absolutely no damage, there's no point. You'll probably just be one-shot anyway. Stat-buffing moves that increase attack (swords dance, quiver dance, tail glow, and the like) give whomever uses them the opportunity to strike back at their opponents. Since they (often) won't even be attacked, their low health is not an issue. Now, they DO have a place. In some stall teams, maybe with perish song or something, the ability to bulk up can be really useful. It's just not a very easy-to-use move, since it doesn't really synergize with alot of more common teams