He’s probably thinking that for anti-Trump purposes he has a slim chance of winning Florida and Ted Cruz doesn’t, so he should stay in the race. The problem is that the odds are very slight (the new Fox poll has him down 23 points) and the price of this slender upside is that he could take potentially crucial votes on March 15 from Cruz in Missouri, Illinois and North Carolina or John Kasich in Ohio (who is now narrowly leading Trump according to the Fox poll). The situation is so desperate that it makes some of the more fanciful scenarios, say, of a surprise Cruz–Rubio tactical alliance announced at the debate, seem more thinkable.