I agree with you, his studio vocals are much much better.
But then we can compare him to most of the infants on the charts today, his vocals are much stronger than 90% of them - and he's at least singing live while most playback.
It's kind like Michael, when we talk about "this wasn't Michael's best day" - that's because we compare him to a certain standard, which is above the "normal standard".
Here's a performance Robin did of Rock With You and Al Green's wonderful song "Let's Stay Together"
I definitely hear what you mean about his falsetto, but on the notes where he's more pushing his voice he really hit them notes well.
http://www.youtube.com/watch?v=CXxjSk54aZg