I always felt that a person who can compose music has to have an IQ level higher than one who just sings whatever she/he is asked to sing by an MD. And if a person can both sing and compose brilliantly, you can imagine his IQ level. So it was with Hemant Kumar.
Punjini- you correctly pointed out the IQ factor. Hemanta was also one of the founder members of Gna Natya Sangha at Mumbai (along with Salil Choudhury).
I believe IQ and emotional intelligence is a must in performing artist. Even with great technical capability – without emotional intelligence one can go only so far!