When your prospective customer opens your web site in a mobile browser and it is slow to download, out of order, is 25 times larger than the screen allows - they will not bother to look at your page. You had a customer in the palm of your hand but you let them go before they even saw your product or service. These days nearly everyone owns a mobile phone and when brand perception and brand liking means so much - not having a mobile site will be a problem.
Having a mobile site not only creates a fantastic experience for customers but it is also a very convenient way that people now shop products and services, you will significantly increase your overall audience loyalty by being available wherever and whenever they want to access your site. A mobile site makes you more accessible to your target audience resulting in what marketers call "lift".
One thing you can be sure of - going mobile is the biggest internet trend right now - if you do not go Mobile you competitors will. Do you want to give your competitor the upper hand?