I think if its important to the customer, then its important to you as well..
having a social presense is very important.... if not for traffic, then for links. you will be building backlinks from social media platforms (i hope) so you can easily add the service of "setting up social platforms".
you can also explain that for every "fan" you get, you are probably getting thousands more views, and with the right advertising (facebook ads is getting more popular now) you can generate alot of traffic from social media.
Social media is also a great place to refer your clients customers, and then there customers can bring in more traffic.
More and more people are using twitter and facebook to virally grow thier businesses...
so DONT stay away from social media.... its very big, and can definately help with your marketing effort...
In some situations social marketing wont be effective, but the same goes for SEO and PPC... you just have to see what works with your client.