Since the only way visitors are going to arrive at your mobile device friendly website is via a redirect from your main site it might seem like their ranking in the search engines doesn't matter. Of course, as with all things technology related, it's not quite that simple.
Duplicate Content issues. Developing good content is hard. You have to pay writers or put in the time yourself. It is much easier to build websites if you copy everything from someone else. The search engines know that so they always look for identical content and try to reward the site that created it.
However, your mobile friendly website is, in fact, sort of a duplicate of the most important parts of your main site. That is why Google's assurance they are aware of the problem is important. They have provided specific direction that sites in the form m.MyMainSite.com or MyMainSite.mobi will be recognized as Mobile device friendly sites and not apply duplicate content penalties.
SEO benefits. The search engines want the best possible experience for visitors so they have said they reward websites which send mobile device visitors to pages tailored for their use. In other words your main site may actually get a boost in the rankings because visitors with mobile devices are being handled in the best possible way.
How many extra customers would you need to pay for your mobile site?
