Social Media Marketing is a social tool, one created to give people the opportunity to communicate, interact, engage and build relationships. What makes social media real are the people who drive the cogs of communication. Take away the real feel, the people of social media through automation and it won't work as effectively for businesses and marketers.
If someone takes the time to send you a tweet or message you, it's good manners that you should take the time to reply to them personally in return. An automated response is a quick way to put someone off in these circumstances and possibly loose them as a prospect.
Breaking Down Barriers
Engaging 1-on-1 with prospects breaks down communication barriers, it works towards making them feel important and special receiving a personal response. Happy prospects equal happy customers coupled with their positive reviews and referrals. Simply put, breaking down barriers goes a long way to enhancing your positive business reputation.
It lets prospects get to know the people behind the brand
Its human nature to want to know who the people are that work for a brand. When a prospect builds a relationship with the people behind the brand they are also building a relationship with the brand.
It enables quick responses
If your social media is automated you will loose touch with the pulse of what is going on with your brand and social media interaction and relationship building. Not personally monitoring your social media presence on a regular basis will prevent you from being available to help people who may have questions about the automated messages you send. Unanswered questions don't build reputations; it leads to disgruntled prospects that may very well leave your brand in favour of one not using automation.
It prevents duplicate messaging
Constantly sending out the same message on social media platforms is not good marketing. You will appear to be a spambot rather than a company someone wants to build a relationship with. By sending out more individual messages yourself you are better placed to avoid duplication and enable relationship building with your prospects.
Automation has a place and can successfully be used in say publishing RSS feeds but the main thing to remember with social media interaction is that behind every computer sits a person with blood pulsing through their veins.
...my two cents worth
It's a lot more rewarding when you work with like-minded individuals who keep you in-the-loop, are keen to share knowledge and who are passionate about turning-out a kick-ass product.