Fantastical brings a couple of other things to the table as well.
First, one app instead of going to two different apps to see/edit reminders and calendar events.
Pre-iOS8, the stock apps didn't do custom repeats (e.g. "on the 3rd Thursday of the month...").
Stock calendar still does not allow assigning a timezone to an event. And does not allow more than two alerts. All of which can be done via OS X Calendar on the desktop and Fantastical on mobile.