What your saying doesn't make any sense.

I think you are a confused beginner.

It's ok

We were all confused beginners once.

You will get better.

Both Buffs & Debuffs are very powerful in this game.

This is why strong players often use both.

+1

And remember your buffs don't require ACC, a struggle for many beginners to get high enough for debuffs.  One of the reasons rares like Apo, Spirithost are so useful in early game

No seriously, mathmatically it makes sense

30% up then 30% down try it on a calculator

The math makes perfect sense, but your dec DEF requires ACC, your inc ATK/DEF does not.  The first GH upgrade I worked on was Magic Resistance, I never realized how much it helped me progress despite all the other stupid noob stuff I did.  Both buffs and debuffs are important.

Does the game calculate like that? Or are buffs and debuffs calculated from the unbuffed stat each (wich I believe how it works)?

And even if you are right and the game calculates the way you think, it's a very strange idea that buffs are useless. Trips allready mentioned why buffs are good. You don't need accuracy to land them.

Another decisive argument: They do different things and can be combined. If I want to enhance my damage, I debuff the enemy with def down. Everybody does that and knows that he has to do that allready. If I want to enhance my damage further, I use an atk buff. Atk debuff doesn't help, it reduces the damage I get, it has no effect to the damage I deal. Def debuff + atk buff do more damage than def debuff + no atk buff. 

Mathematically proofed that your "buffs are junk"-theorie is wrong, q.e.d.

Also, as a side note, your assumption is that buffs/debuffs apply multiplicatively - IE if you have a 30% DEF down and 30% DEF down, you assume the equation is X * 1.3 * 0.7 = 0.91X.

I'm not sure if that is truly the case. It may be that they are additive, in which case the two would just cancel out.