There is a lot of talk at this point about the desirability of Republicans who don't want Trump to be nominated uniting around a single candidate. One problem with that proposal is that the candidate that the establishment wants to unite around is at this point Rubio, while the only candidate anywhere close to Trump in delegate count is Cruz.
But there is another problem. Suppose all but one of Trump's opponents pulls out of the race. Some of their votes in future primaries will go to Trump. He is currently getting close to half the votes in primaries—with those extra votes he might go over half. That could easily result in his getting to the convention with a majority of the delegates, at which point the game is over.
The analysis is complicated by the fact that some primaries are winner take all. For those, Trump is better off with multiple opponents splitting the vote. But for primaries where delegates are apportioned to candidates roughly in proportion to the votes they get, multiple opponents result in fewer delegates going to Trump. And the case of winner take all primaries is complicated by the fact that different candidates are popular in different states, as yesterday's results make clear. If Rubio drops out, Trump takes Florida. If Cruz had dropped out, Trump would have had a plurality in Texas, and there may be future winner take all states with the same pattern.
Of course, even if Trump doesn't arrive at the convention with a majority, he may still win. Christie is the first major figure to switch sides but may not be the last.