It's not a silver bullet, it's only a model of the problem. If you profiler tells you some code is slow, you can model why it must be slow by using big-O. In fact it's the standard way of explaining such things. Without it, you must spend your time babbling about special cases.

It's like, yeah, you don't "technically" need to know any 2+ syllable words to be a programmer, but you're really not helping yourself by avoiding them.