When you spend money developing a mobile app, you have to future proof it so that keeps up with the apps/sites of similar functionality, otherwise people will end going to your competitors (obviously that statement depends on your service, but its certainly the case in my sector). People with smartphones wont want to use a site that is so stripped down it would work well on a crappy handset.

We are sitting on the edge of a large explosion of smartphone devices and (potentially) users (with lots of Android handsets coming out this year). In the UK you can now get a Android powered device for £20 / month with a free handset. If you want a smartphone (and therefore probably interested in using the web), you will be able to afford one this year (at least in Europe!). This is going to be huge.

The iPhone is the standard they all will judge themselves against, and therefore the iPhone is the benchmark device.

I would suspect that the 50% smartphone / 50% other phone traffic statistic will be way higher towards smartphones in 12 months time.