Like John said. Seeing the colors is an immediate tip-off. In the web view you can also show/hide other calendars that you have access to, so if I need to check a particular block of time, I can just show the other calendars to see if something will fit.
Even with calendars, by immediately sending an invite, the other people have to look at that time block and decide if it is really free, or they forgot to put something on their calendar.
You also control your notifications, so if they accepted you can get an SMS notification, or the attendee list in the event gets updated with responses.
