Ever since Lost went with that whole 'Lost experience' thing last year, it seems like all the shows are jumping on with online extras, Heroes being one of the worst offenders.
The comics aren't really essential, I don't think...some of the time they give neat background stuff, like the Hana series, which tells her whole backstory, or last week's, which tells how Sylar got from Texas to Zane's in Virginia. But mostly they're just side-stories, like Micah getting bullied or Nathan saving a woman from a fire, that kind of thing. The only time they ever put 'essential' info in one, I think, was when they showed why Sylar stayed put in his cell after Eden's death...I know a lot of people were confused when he was still in the cell the next week on the show, cause they didn't really show any reason on tv why he shouldn't have escaped.
So, to sum up, the comics are a nice addition, but not really vital material, for the most part. Even if you can't see them on NBC's site (which does suck, yes), there's probably a summary of them at least on the Heroes Wiki...I don't know for sure, never having needed to look, but I would think they're discussed there.