When do polls matter?

A year ago, many if not most of us argued that primary polls were questionable at best, because, (1) traditional polling methods were inaccurate and slanted towards an older demographic, and (2) Bernie simply wasn't well enough known for people to have an opinion on.

And just as we've seen, #2 has been utilized quite effectively as a major campaign tool. Keep Bernie out of the press as long as possible, but once you must eventually talk about him, make sure to minimize anything of substance (what he's done, what he stands for) in favor of tabloid-style gossip.

Now we have polls showing Hillary consistently slipping further against Trump, while Bernie maintains a steady lead. But of course, now they are the ones arguing against the validity of these polls.

Why? Are the methods used not accurate? Do people not know who Hillary or Trump are?

Or is there something else? Like an expected bounce after she's found innocent of any wrongdoing from the private server issue (similar to the bounce she got after the Benghazi hearing)?

Please help me understand why polls from a year ago were valid to predict today, but polls from today are too early to predict five months from now?