Agreed. As long as Newt stays in he will clip more delegates from Romney, and will have more leverage at the (hopefully open) convention.
Ohio shows the folly of this reasoning. If Santorum had Newt’s +15 in Ohio, he would have won the full slate there.
Newt is dividing enough votes and letting Romney pick up the votes that he needs to win the nomination outright - no brokered convention.
Newt, after tonight - cannot force a brokered convention. Romney and Santorum won too many delegates.