Frodo, with the Ring, would have been more use to Sauron alive than dead.
[In reply to]
Frodo would have eventually turned into a wraith, and become under the control of Sauron. Presumably, it would then have been fairly easy for Sauron to obtain the Ring. Had the Witch-king killed Frodo, then there's no guarantee they would have got the Ring. There's still 3 Hobbits and 1 Man on Weathertop.
Gandalf also says:
If they had succeeded, you would have become like they are, only weaker and under their command. You would have became a wraith under the dominion of the Dark Lord; and he would have tormented you for trying to keep his Ring, if any greater torment were possible than being robbed of it and seeing it on his hand.