It varies depending on the subset. It's not like "people on the Internet" is some homogenous mass that you can make this kind of generalization about (at least...in this case.

)
People who refuse to buy 3PP's and only buy WotC are underrating them (and those people are on the internet).
People who think that the 4e GSL (or 4e in general) was made specifically because WotC wanted to stop loosing sales to 3PP's are overrating them (and those people are on the internet).
People who think that all 3PP products are crap based on the early d20 splurge are underrating them.
People who think Paizo is a genuine threat to WotC are overrating them.
People who like some 3PP's, but not all, and look before they buy, are probably rating them just right.
But you've also got people who are overrating or underrating WotC's importance, too, so it's not like it's a bunch of OGL zealots who are blazing a unique and special trail, either.
3PP's are worth checking out, but not every product is good, and they don't have a big enough presence to intimidate WotC into doing anything (though they might make the few people involved in them some small profit if they're solid enough over the long term).
That's a pretty balanced view, and that's all on the internet, so...
....really, depends on your context.