First up: a quick disclaimer. I'm a fan of Bernie, but will not be voting for him in the primary, so make of that what you will.
In the era of nearly unlimited campaign spending, almost every politician who wants to compete will likely have some sort of super pac associated with them. Hillary has PrioritieUSA, and there are innumerable billionaires backing random GOP candidates - so much so that it's not even really worth naming them all.
Bernie does not. This is both a virtue, and a serious, serious problem, but not in the way that many tend to think (with regards to money, donations, etc.). Follow below the squiggle for more.
Much of the conversation around SPACs revolves around their capacity to accept unlimited donations. Reviled on the left, SPACs are nevertheless increasingly necessary in an era when almost unlimited amounts of money can be spent on elections. Bernie's conviction to not having a SPAC puts him outside the realm of this political reality, which serves him well, and should be commended.
However, despite the possibility that Bernie may have trouble staying in the race without some SPAC backing, that's not the real concern.
More importantly, there's nothing stopping people from setting up a SPAC for Bernie. In much the same way that SuperPACs have proliferated on the republican side, there is a danger that some well meaning - or potentially ill-meaning individual will set up a SuperPAC to capitalize on Bernie's popularity amongst the base, begin taking donations, and then sending out ads, messages or other paraphernalia on his behalf, but without his consent. If all goes well, this won't happen.
But if someone takes the initiative - say a brother Koch or other Republican Donor, Bernie may find himself in the awkward space of having to argue against ads coming from an outfit that nominally links itself to him.
Because Bernie is not interested in SPACs, he will be pushing people to donate directly to his campaign. There are limits to those contributions however, and donors who really appreciate what Bernie does who will want to go beyond those limits. If Bernie sticks to his guns, however, he will not have anywhere to direct people. What happens to that Hollywood type that wants to give Bernie 10k? The union in SC? Do they leave money off the donation table - or find some other way of spending it to support him?
More importantly, we're already there. Question is, is this as far as PACs will go to support him, or will we see some set up that don't have his interests or ideology in mind?