The conventional wisdom is that we need to reduce the primaries to a two-man race to stop Trump. The question then is whether Cruz or Rubio should drop out.
We need to keep the long game in mind. This is a two-step process. It's not enough to defeat Trump; you need to defeat Hillary.
For Trump to win the general election is a worse-case scenario. For Hillary to win the general election is a worse-case scenario.
The question at issue right now is not whether Rubio or Cruz can win 1237 delegates in the remaining primaries, but whether one or both can prevent Trump from winning the magic number. Trump is still opposed by about 60-70% of primary voters. So he's very vulnerable.
At this point, Cruz probably has his best states behind him, whereas Rubio has better states ahead of him–if he can win Florida.
I realize there are Republicans who can't stand Rubio on immigration, but if Hillary wins, it's not as though she will be any improvement on illegal immigration.