I found myself squarely on the side of the DNC during the campaign last year, because the argument that you should be a democrat if you want the democratic party to support you sort of made sense to me. It's a private organization, so they can make whatever rule they want. I would argue, and did, that the GOP has the same prerogative, and that they could have used that to keep the country safe from Trump.
I'm not so sure anymore. It seems more realistic that there aren't any realistic possibilities of someone becoming the president (and really very little chance of even becoming a legislator) if you don't run as a democrat or republican. In that sense they aren't really private. They're de facto government. So I've somewhat come around on this. In any case, no matter what you think of Clinton or Sanders, I'm not sure now is the time for the democrats to piss a lot of their potential voters off.
I can and will spend significant personal funds this year to try to get democrats elected. I've hated the GOP for a long time, but I've never donated cash before. I'm not donating to the DNC, but to individual candidates. However, if the DNC fucks up this opportunity this will be the first and last time I spend hard earned cash on their bullshit machinery.