You are mistaking Berserk in FFX with Berserk from all the other FFs.

In FFX, Berserk provides no bonuses to your Strength rating, and due to the strategic nature of the game, which is based entirely on taking your turns and using them to the fullest, you need that turn to be able to pump out as much damage as you can by any means possible.

This usually does not involve Berserk. Berserk also is not player castable, and is enemy only, and MOST (not all) of the time, Berserk is cast in tandem with other malicious ailments, such as Poison and Confusion (Bad Breath, anyone?). However, Dark Elementals and Black Elementals can cast Berserk alone.

You Int. and PAL players are really missing out. It's FUN watching your characters beat the living snot out of each other while dying from Malaria.

However, FFX-2's Berserk is far more useful.