Tunesmith, I don't know that Roy Orbison's voice was ever regaled for its "power." I think it was mainly his pitch range, tone, and overall quality.

Also, there's a HUGE difference between having a "whisper" of a voice and merely not being able to project one's voice in a huge venue or above a cacaphony of other sounds (such as in a pub) without a mic.

I compared acoustic guitars to pianos and trumpets earlier. A prime example I think would be this:
I heard Andrès Segovia in concert back in the '60s in a large concert hall. He absolutely refused to play unless and until the audience bloody shut the hell up -- totally!   They/we did -- this time -- because when he had appeared at that same concert hall earlier on, and the audience wouldn't be quiet to listen, he had walked off the stage.
Even with everyone being quiet, his classical acoustic guitar did not "fill" the room the way violins, brass, pianos, and many opera singers (not to mention rock belters) can.
But to denigrate his talent or the beauty of his music because of that would be ridiculous.

There are many singers -- and some instruments -- that can be heard just fine in small auditoriums with good acoustics or in small clubs, when the audience is listening (as opposed to trying to yak over them).   But to be heard over an entire orchestra or in an arena or over the din of your average pub without a mic, that takes special lung power.

I'd say range of volume is ONE criterion of being "a good singer" -- but neither a necessary nor a sufficient one. : )

And I'd far rather listen to Roy Orbison than the vast majority of big-voiced professional singers I've heard.