1. He probably doesn't have the legal standing to pursue the case in the first place.
2. It will send a clear message to voters that Donald Trump is afraid of Ted Cruz, which is the worst message you can possibly send if you are running as a bold, fearless, in-your-face candidate.
He has more than Schwartz does. Trump is an opponent in an election contest.
Above i remarked about filing an amicus brief - all a form of "publicity," to force the candidates to take clear positions in the eye of the public. But perhaps Trump could be joined as co-plaintiff, arguing ONLY the standing, etc. procedural aspect.
I think the court still tosses the case. The only difference is the calling Cruz's bluff on "I want to get a ruling from the courts."
Actually, Trump would have standing since as a presidential candidate, he could claim a particularized injury from the presence of an ineligible candidate siphoning votes which Trump could claim in part would otherwise go to him. Polling this morning showing Trump wins every head to head challenge against the other Republicans, save for Cruz, could be compelling proof.