It's true, the content is more important than how it's written...but there is also the problem that some are so badly written you almost can't find the plot under all the misspellings, typos, grammatical errors, etc. And a good plot can be ruined by bad writing. Also, it's easier to read if there are paragraph breaks, etc.
I'm a member of a fairly large fanfiction site on the net, and the subcategory I write for happens to be one of the largest in its category. When I go to read what others have written, sometimes I find myself sitting there reading aloud, depending on the sound of the word to guide me to what the word actually is. English is one of the hardest languages, and it's mostly because a lot of English words aren't spelled the way they sound. And the juxtaposition of 'they're' and 'their' isn't going to be picked up by a spellchecker. So beta readers are a good thing to have; very good. They can catch the typos, and if they happen to know a little about the topic, so much the better, they can suggest things that wouldn't occur to you, or pick up on continuity flaws. My beta-reader just picked out a major continuity flaw between chapter 3 and chapter 32 of my novel, and if it wasn't for him I would have sent it off to the publisher like that! Beta-readers are a must for anyone who writes, whether it be a novel or a ficlet.