iCal support is definately important, it's great that you're putting it in. People will be able to sync their laptops, PDAs, phones with a JEvents calendar directly. Where supporting an API like Google's is important is in distributing data more widely, perhaps to several different websites including Google.
Google Calendar is very important, many people already use it to manage their time and have already set up to synchronise their calendar application with it, often paying a small amount for software to do it. They often won't be able to link up with a JEvents website as well, either because their software is not suitable or because they don't have the knowledge. Supporting Google's APIs would allow a user to modify an event on a JEvents website and then sync their mobile device with Google immediately. Also, other JEvents websites, also accessing the same Google Calendar, would also immediately show the change.
Supporting Google API would also be easy for website admins to set up and manage, only requiring the calendar identifier and access criteria. Many people will struggle to set up cron jobs properly and safely on their websites and some simply won't have that facility.
So there you have it, supporting Google API would be great for disseminating data without latency and for ease of use! You must be convinced now