That's a pretty simplified statement, and I can't agree with that. I've heard plenty of amps that will get loud enough, but then run out of steam on difficult passages. Dynamic headroom is very important IMO, and I'd personally rather have more than enough power than not enough.
For example I used to own a pair of ESS AMT 1Ds, with the famous Heil Air Motion Transformers. I powered those speakers for about a year with an NAD 2200 stereo power amp. That amp was conservatively rated at 100 wpc, with pretty respectable dynamic headroom. I thought it sounded great. But when my stereo sales rep offered to bring a second NAD 2200 by and run the pair in bridged mono mode, I was floored at the way the speakers opened up. That was 400 wpc in bridged mode, and I would have thought complete overkill for my ESSs, but I was wrong. I did buy the second NAD on the spot BTW.