In the last time i often see websites advertising their mobile app to users on mobile devices. But just because i visit your website with a mobile device, i don't want to install another app just to look at your site. Seriously. I'm just looking for the information right now and I accept that the average website does not look perfect on my phone. If i like your site, i will come back to it on my desktop pc.
In fact, most of the single purpose mobile apps are simply crap. They don't have a unique feature that let me think: "Oh, thank God i have this app". They often just present the content of the website in a different layout, which is really worse in a user experience view and - to be honest - they are magnitudes slower than the web browser app. Furthermore, they often lack support of the most basic features every mobile web browser has: Copy&Paste of all parts of the page, Bookmarking, Back/Forward, Tabbing, etc. We just need a mobile app if the website contains features which cannot be fullfilled with a decent mobile website (lots of local storage for example).
Although it takes not much to convert your one time mobile visitors into happy returning visitors: create a readable and navigatable website for touch-enabled devices. HTML5 gives us many interfaces to interact with the device and lets us interact much better with the user. CSS3 brings us much more ways to design the layout: Media Queries to change the appearance based on the view size of the browser, and hardware-accelerated effects and styles.