A David Duke endorsement won't help Trump, an endorsement by Ted Cruz might be of some help. Not necessarily by getting a ton of Cruzers (as opposed to ex Cruz supporters) out to vote, but in showing Trump can unite and be gracious he could sway some votes.
I am for him getting as many votes as he can, I don't think the support of Cruz will hurt at all, and will likely give a slight bump (maybe 1%?), perhaps the bump that will win this thing. I no longer particularly like Ted, but I'm all for him doing what he should have done months ago.
I agree with you.