Michelle,
You don't even need the form; if you aren't collecting more info on your form than you already have, then allow your leads to register for the webinar with one click. You can change their status to "Registered", and Marketo will pass the info right to the webinar provider.
As Tom said, YMMV, so I would test this out on a subset of a list before trying it across an entire campaign.
On the other hand, you lose the ability to use progressive profiing on the webinar LP form and the opportunity to gain more information about your leads.