Well, Power Infusion is a strange one. In my experience (as a mage) Power Infusion tends to get thrown around without a whole heap of strategy, and in the occasions that I was "gifted" with it, I was either completely not expecting it, or it came at a time not condusive to extreme damage, or I couldn't take advantage of it, due to 3+ consecutive prior crits or something.
A mage or warlock is extremely aggro aware, and indeed - we try to push the line as far as is safe. With that in mind, it's often useful for the nuker to judge when is a good time to request a PI type buff.
However, and this is a big point - the single best use of PI I have seen is on druids and paladins during a priest class call at Nefarian. People tend to forget about the healing potential of this talent.
Good luck with developing - I hope some of my thoughts have been helpful.