From what I gather he can perplex his values when he's on defense. Is that the only way this power can be useful?

I'm assuming you're referring to the Doctor Octopus figure from Web of Spider-Man?
It always helps to post the power in question.
He can use perplex 4 times, but each time he has to modify one of his own values. So you could perplex up attack twice and damage twice, movement once and defense 3 times, or any other combination you can think of. You could even perplex up the same value 4 times, but it would only matter if there was also a penalty somewhere.

You can use it like any other application of Perplex. When he can't attack, yes, I'd bump his defense up by 4 (you can only get the +3, but the extra is to counteract a -1 someone might throw your way)

But you can use it to bump up any value: range, attack, movement, damage

They all still adhere to the Rule of 3, though.

Great against Mr. Fanatasic's -2 perplex. +4 defense and +1 with hindering -2 = +3. Yeah 20 defense!!

But as for the intiial question, his power is ALWAYS ususefull