In addition to having too little battery life and being significantly larger and heavier, iPhone is not nearly as sturdy, water resistant, nor will it be as easy to operate. That you can spend a few hundred bucks to add HR and sensors (or $750 to add this stuff and measure power as well) does not change this. Not working where there is no data signal is a big deal. Plus the iPhone costs a lot more.

Android and iPhone will never represent serious competition to the cycling computer market even if the batt problems are solved (this won't happen because these devices need to be more powerful). A phone touch screen is really a lousy way to control things. If you get something that has sensors, the price starts approaching a real computer fast. The size, weight, and fragility are inherent to the devices.