Senior Executives Online

Should senior execs of large enterprises like Wal-Mart, be on Twitter?   Some interesting thoughts.  But is this an example of attempting to micro manage consumer interaction?  Sure they need to know what is going on in Social Media, whats trending in their areas of interest.  But do they really need to answer every message, every idea?  Isn't this what your employees should help you with?  What PR firms do?