They are right man.
The Glaring slash + spirited roar + crit from behind + RP (dmg up) = Colossal DMG.
Then, without the "Glaring Slash" you lose a large portion of the total damage you could do.
Maybe if you fight against inexperienced players, you will have enough damage anyway. But in many cases you will fight with players who have "props" so the "30 props" that the glarin slash gives you are vital for high damage to the many tanks that abound in the game.
Besides that, in case of fighting against more fragile classes, you can make a 1 HIT KO easily.
As for "penetration" buf I do not see it useful to go up, since the difference between lvl 1 and max lvl is 10 pts of dex. That's why I think it's better to use skill points in other skills. But in the end it's all up to you.