This is, IMO, a gross overgeneralization. Remember, wattage is just one input into volume. If you are using a low SPL cab (i.e. a cab tuned very low with lots of deep bass response), an honestly rated 1000 watt head is not going to be as loud as an honestly rated 500 watt head into a more normal, mid voiced, higher SPL cab.
Also, remember, doubling the wattage of an amp, all other things being equal, does impact the volume significantly, but not as much as you might think (the oft quoted +3db). What it does do, besides push lower SPL cabs, is also provide the headroom before clipping for aggressive playing styles that still require a clean, undistorted tone (like slap style playing). A higher wattage amp (all other things being equal again) will result in a much more open, deeper, less compressed low end for these types of playing styles.
In general, I agree with you, but there are many situations where a very powerful amp will make a big difference, and it has very little to do with overall volume.
IMO and IME. I am lucky enough to enjoy the more mid presence and tighter low end of these higher SPL type cabs, so I never get close to running out of juice with a modern 500 watt or so SS amp. However, if I used cabs like the Acme's or the older Bergantino HT's or EA VL's, it would be a different story.