That's one of the questions Scott Oser poses on Acronym, and it's a good one. If "free" means it's just a sales pitch, that could be fine if I know that's what it is and that's what I signed up for (sometimes I do want to learn about a product, and an infomercial comes in handy). If it's marketed as educational, though, and ends up being a sales pitch, well, you're going to get a tee'd off audience. Especially if, as seems to be increasingly common, your audience ends up getting on a never-ending e-mail list that bombards them with more pitches after the fact.
Scott also asks if these free vendor webinars compete with paid (or, I would argue, free) association webinar offerings. Sure, they do. Everyone's time is limited, even for freebies, and if there's a chance you can get good info for free, you'll likely try it first. If the experience is good, paid webinars will suffer an audience drain. And if the experience is bad, that could tarnish even good association-produced webinars in an audience member's mind (i.e., "all webinars are boring/promotional/not worth the time").
Once again, I think the key is to be truthful and transparent. If you're offering an infomercial, be clear about what it is. Let people know if they're signing away their in-box in perpetuity if they sign up (but how stupid is it to bait and switch people like that? Can you think of a better way to get someone to never want to hear of your product again, much less buy it, than spamming them endlessly as a reward for signing up to hear your pitch to begin with?). But there is a market for infomercials, so I wouldn't count them out altogether. Just be honest and smart about marketing them as what they are, and respectful in your followup.
If you're offering good, unbiased education that you think is worth paying for, be truthful and transparent about that. There's a market for that, too, and I would argue one people still are willing to pay for if they can. As Jeff Cobb points out in a comment over on Acronym, the rub is that more vendors and other third parties are getting smart about their freebies and offering real education, not just infomercials. That competes head-to-head with what associations are doing.
Though I hate the word, this could be a place where "coopertition" comes into play. Can you work with these third parties somehow instead of competing with them for the shrinking pool of webinar dollars? I don't think this is a trend that will go away. In fact, I expect to see more of the smart marketers offering free webinars that establish them as thought leaders in their field and fewer infomercials. Associations may have a certain amount of built-in loyalty, but I wouldn't rely on that too much. Whoever has the best info will win, especially if it's offered for free.
It's a Web thing, and we're all struggling with it: Having given our best stuff away since the Intertubes started, and with more free stuff added to the pile daily, we have a hard time finding ways to make people pay for what they're used to getting for free, especially when the quality is similar. Whether you're talking about newspaper classified ads v. Craigslist or associations' webinars v. vendor free webinars, we're all in the same bucket on this one. And I don't think quality alone will make it worth buying, unfortunately.
Update: Great discussion on this whole deal by Ken Molay at the Webinar blog. I love this line: "Value justifies cost… If the value of your exclusive content surpasses the cost to the attendees, they will pay for it. But if you are charging them to get content they could just as easily get from other free sources, watch out." The key is to figure out what will make it that valuable. Thanks, Ken.