Compare these two builds:
A) Telekinesis 10 (Temporary Disadvantage, Berserk (N/A), -25%).
B) Telekinesis 10 (Accessibility, Only in berserk state, -X%).
In the case A, you are always in control of your own TK, but you always fall in berserk state whenever you use your TK. In the case B, you must be in berserk state as a prerequisite for being able to use your TK. The two cases are slightly different, but since anyone who has Berserk can deriberately drive himself into berserk state (p. B124), the cases A and B are quite equivalent. I'd price the "-X%" at -25%.
By the way, there's a murphy on psi abilities and Berserk: When you're berserk, your possible maneuvers are restricted to All-Out Attack, Move, and Move and Attack, and nothing else (p. B124). However, most psi abilities require Concentrate maneuver, except for abilities that touching, which use Attack maneuver (
Psionic Powers, p. 22). Therefore, when you are berserk, you can't use most psi abilities. This is easy to resolve -- simply ignore the text on p. B124 and just allow Concentrate.