An Iphone app is just another touch point for people interested in your content, products or services to connect.
In principle you want as many touch points as possible, to maximize how people can find you and connect with you. Popular touch points are: telephone, websites, facebook fan pages, myspace pages, YouTube channels, etc.
In deciding if you want to add an iPhone app to that list, you need to examine the cost / benefit of such an exercise: how many people do you think will use the app? How much money will that generate? How much will it cost building the app in terms of money and the time you have to spend coming up with it and maintaining the app?
If you find that the return on your investment is greater than the cost, and that there is no other way to make more money at this point, then investing in an iPhone app is a great idea.