Do these online language classes give you feedback from live teachers? Otherwise I can't see how they can help you learn how to communicate in the language you're studying.
(Ooops. La Isla's reply above appear while I was writing this. Anyway....)
I'm not sure I understand your question, La Isla? You could make the same comment about a traditional textbook (which doesn't include audio). Even without a teacher, a self-motivated person could learn a lot just by pouring over (and over, and over again...) the 501 Spanish Verbs book, including the grammar sections, or course. That thing is like the Bible.
There's simply no avoiding it. At least for me, a lot of language learning is exposure/exposure/exposure, repetition/repetition/repetition, and memorization/memorization/memorization. Until some things finally start to sink in.
Nothing replaces a teacher, of course, or exposure (ear/accent) to spoken, conversational Spanish. But some of the better on-line sites prove valuable to me also, as a supplement. They're just another tool in the learning toolbox.
And I think that some of the fancier sites also include (for a cost) access to video/audio conferencing with a Spanish speaker. (Hello Skype and whatever else.)
Like I said, I use on-line sites as a supplement. I personally get lots of feedback since I attend traditional classes with a teacher and live in that larger classroom.....México!