It depends on how far and often you ride. I've always been using my iPhone as my GPS / biking computer using Strava and MapMyRide.
While iPhone is great to use and track your rides, the battery drains really fast - more than half after my regular rides, which are about 2 - 2.5 hours, and that is while running in the background.

That being said, if you plan on riding for short periods of time (under 3 hours), you might be fine with your iPhone as your cycle computer.

But, if you are planning to ride longer, or if you want to mount it on your bike to have it constantly on, you need something else.
Having it on all the time will probably drain your battery completely in under 2 hours. Plus, you don't want to drain your battery and be left without your phone while riding.