As for talent as a singer, vocal power (being able to fill a large concert hall sans amplification) is just one of several important factors.    Range of pitch, precision of pitch, ability to "carry a tune" (which is not the same thing as just being on pitch), tonal quality (resonance, etc.), phrasing, interpretation of the songs, connection with the audience -- all these are important aspects of being a good singer.   Since people don't always have to sing live in large rooms and since excellent vocal amplification tools are pretty standard these days, I'd say that sheer vocal power (volume) is maybe the least important.

If you don't care for Bublé's music, fine. But if his unamplified voice is rather soft (a "crooner"), that seems a poor justification for dismissing his talent as a singer altogether.