This question is almost always asked when we recommend building a mobile site in addition to a standard desktop site. The issue is even more important today as it is almost certain that Google is basing its results, in part, on whether or not URLs have a mobile site.
The reasoning is logical! If you provide a mobile site then you are making yourself more accessible to more individuals, therefore you are more relevant. The leading statistic driving this trend is that sometime last October, mobile devices became the preferred way we browse the Internet.
So, help out your site and your audience and make sure you are mobile ready!