Trump needs to somehow be trumped. He could singlehandedly give Hillary Clinton the keys to the White House if he doesn't shut up. The Republicans don't need any help at sabotaging their own campaigns. That, and there's talk of him considering running as an Indy in the general if he doesn't win the GOP nomination.
Might do well for the establishment to stop attacking him then and setting him off. He's speaking a real truth about immigration, I couldn't care less what his opinion on the birth certificate is.
The press, in this case, invited him to talk about the birth certificate, and he's not making it a focus of the campaign this time, and he's said that 100 times at least.
Trump probably won't get the nomination, but if it helps move the field in the right direction on amnesty, I'm all for him staying in the race as long as possible.
If he pulls a Perot, it will be the establishment's own fault for abusing conservatives.