Well, I know a lot of people in the liberty movement have a problem with this, because it's not exactly intuitive.
So allow me to explain...
Working for Mitch, Jesse gets access to all of Mitch's lists. He gets access to all of his fundraisers. He gets access to all of his big donors. He gets close to Mitch himself. Even if Mitch doesn't endorse Rand for President (I'm betting that he will) the access to these insiders is huge regardless of whether or not we like it. This will give Rand a tremendous advantage in 2016. And, if Mitch openly endorsed Rand then he could possibly be considered the most likely candidate for victory.
It's not something that I would personally do, I couldn't stomach it. It's dirty work, but someone has to do it, and sometimes winning involves doing things that are unpleasant. Jesse takes a lot of heat for things that are necessecary but unpopular.