It amazes me how marketers actually think that the user is that stupid to believe that Webinars have limited spots. Do you want to sell exclusivity? Do you really want to limit the number of potential customers you might get? I’m sure you can do better.
For example: a reward to the first ones to register, exclusive and never released downloadables only for those who attend and stay till the end, or highlighting the fact that you won’t be recording the session and sending it afterward.
Respect your customers. They are smart people.