Berserk can be very useful if used correctly/in the right situations.
The spell increases all outgoing damage by 30% while increases all incoming damage by 40%. That being said, it's easy how others who aren't too familiar with the spell would describe it as a "waste" or as you say "a disadvantage spell."
However, the spell is ideal if certain situations about your wizard are met:
Your resistance is high. Your health is high.
That's it. If you're able to tank attacks, then the extra damage won't be too much to you.
So, how is this spell useful?
Well, I use the spell on all my wizards except (for obvious reasons). The spell helps to close that large gap schools like ,and may have in terms of attacking. Since these schools are relatively good at defending itself, the spell is ideal as it can turn attacks into devastating ones without the use of a single blade.
Mob Battles - Excellent here. Using the spell just before attacking is an effective way to use it. It minimizes damage and maximizes yours.
With AoE's - I can't emphasize enough how useful the spell is in powering up your AoE's spells (granted it increases all outputting damage). This combined with enchanted AoE's can dispatch enemies quickly. Especially if/when you land criticals.
So, again, the spell is ideal if used correctly. It's not wise to use it at the start of boss battles for example.