This was at the height of her backlash (of the SEX book, Body of Evidence, and the Erotica album) hated by America and live on National Tv so she was understandably a bit nervous, still she managed to put out a decent vocal performance which she didn't need to, since at this point she had nothing left to prove...
Madonna doesn't have the best voice by any means, but she's a competent singer and knows how to use her voice to her benefit (rather than writing songs that are totally out of her range or whatever). That's what matters imo.
It's not weak by any means. Have you listened to Evita?
Anyway, I find those belting voices the most annoying to listen to. That's why I can't see shows like "the voice" with all those people trying to outdo each other. when you listen & close your eyes all I hear are screaming cats/idiots. nothing annoys me more. Give me Madonna's beautiful warm, and non-try hard angelic voice any day over those people who think screaming like christina wannabes equals quality. Sometimes it gets so bad these people don't even sing the song or sing the lyrics, it's all about trying to out-do each other. It gives me a headache.
People like Bob Dylan and John Lennon also have/had a softer like voice like Madonna. I find it more poetic. Simplicity goes a long way.