The company wants you to sign up for a Pro account because, primarily, it needs to support itself. It seems they did consider other options like ads, but the team did not "want to hurt the Pushbullet experience with ads or betraying your trust by selling your data. Since Pushbullet must support itself, we want to do that by having you as our customer."
Interested customers can sign up for Pro account right now. However accounts won't be charged until December 1st. At that date features like priority support, Universal Copy & Paste, mirrored notification, unlimited SMS won't be available for free accounts anymore. In order to access those features, users will have to pay either $4.99/mo or $39.99 per year. Otherwise, sending files size limit will remain at 25MB for free users while Pro accounts will see this limit boosted to 1GB. Storage space for current users is 2GB. Pro users will see this number jump to 100GB.
As I said in previous blog post, Pushbullet is my favorite file sharing app and it's cross platform. Unfortunately, I do not have any professional use for it, so it is very unlikely that I'll sign up for a Pro account. But I encourage heavy users or corporate customers to sign up for it.
Check out Pushbullet blog post to learn more about the Pro account benefits.