That... kind of stinks, actually. If your free version doesn't offer the value proposition that makes people excited about your service in the first place, you should perhaps rethink having a free version. It's like giving away popsicles without the stick.
or, depending on your point of view, sticks without the popsicle :)
Do both parties need the professional license? What if I want my clients to be able to send me their documents.
The sender needs a Pro account for the transfer to be secure. If a Pro account sends to a non-Pro account, the transfer will still be secure. More answers at http://wireover.com/faq.