i've got a quarter that says it wasn't the testing but the fiddling with the water composition afterwards that was so detrimental. with a few exceptions, a consistent water composition is better than a fluctuating. oscillating pH levels, quick-fix ammonia treatments, high then low salt levels -- all these stress fish and weaken their immune systems. the best thing one can do for your fish isn't have perfect water but just clean water. if you freak out and change everything the instant something is less than ideal, then yeah, testing is a form of overkill. if you just use testing as a supplemental to know whats going on between regular water changes, then it can be quite helpful.
its not the information, its how you use it.