What are the benefits of being a Pro / Pro+ subscriber?

There are a lot of them. Among the most important are:

• more Sites to create and more storage space
• SSL for all your Sites (available in Pro+ plan)
• higher limits of members for all your private Sites
• advanced web statistics for all your Sites
• configurable visibility of your profile
• no adverstising on your Sites

You can compare the plans at http://www.wikidot.com/plans