I don't battle competitively, but do enjoy the casual battles with friends and building pokemon to use.
However, I've never much used stat altering moves, such as swords dance or the like. It just never seemed worth it when I felt like I was wasting the chance to just hit them with something offensive, or possibly something which would cause a status effect which could give me an extra turn(I.E. Sleep, Paralysis) and make out just as well/better.
Tell me why I'm wrong.
Here are some cases in which stat altering moves are really beneficial (maybe to the point where they can win you the game, depending on circumstances).
1. You're up against a really bulky Pokémon that can heal itself more than you can do damage.
Usually, these Pokémon will carry some form of status, like Thunder Wave, Will-O-Wisp, or Toxic. If they heal more than you can dish out, and they have this damage over time status on you, then you're bound to lose (especially if you're a physical attack and are burned). Perhaps they T-Wave you instead, but then you have a 25% to not move, and they will sometimes be able to attack you. Even if they don't carry status, if they heal more than you deal, then you're not getting anywhere. Keep in mind that depending on how much damage you deal, they might not even have to heal every turn, so you might up losing the PP war as well. Being able to set up means you might end up dealing more damage than they can heal, meaning you can break through that Pokémon.
2. You're slower.
A lot of Pokémon have low base speed but also have access to Dragon Dance, which boosts attack and speed. Just by the description of Dragon Dance, you can imagine, what a faster and hard hitting Pokémon can do to your opponent's team. Sometimes if you set up, you just win right off the bat. Some Pokémon already hit super hard, but lack the speed to sweep, so Rock Polish or Agility would be good moves for them to defeat their opponents.
3. Calm Mind.
Calm Mind boosts your SpAtk and SpDef, so you deal more damage and become more resistant to special hits. There are so many times when Calm Mind can win you the game as well. You don't have to be faster, your opponent just ends up not being able to kill you.