Right - except there's a law of diminishing returns; if you're playing with that Twin in a tiny club (or at home, or in an apartment) it's going to sound like crap. I LOVE the sound of a Twin being run hot enough that the sound "blooms" (as described previously), which can be clean as a whistle...but most players do not know how to make adjustments on amps and tub amps just are not "plug 'n play" - you HAVE to do some tweaking, and the same settings will not work in all situations.The Twin is used NOT for volume but for extremely clean headroom and overall performance.
The rules don't change just for steel guitar - many, many 6-string players have an "amp stable" of 3-4 amps, and players who dabble in more varied styles (like I do) might have a dozen. Each has a particular tonal purpose, or handles a particular type of room better than another. One high-powered amp is extremely limiting IMO - you'll sound great at an outdoors festival with unmiked amps, but at home you'll wonder where all your tone went (and you can stare at your hands all you want - it's not in there either... ).
High power and stiff, clean-tone speakers work great at higher volume levels, but are not so good a low levels. And while I can warm up a high-powered amp for somewhat decent sound at medium volume or clean up a low-powered one for higher levels, I fail to comprehend how any player really concerned with decent tone can use just one amp in all situations.