This question is raised by my friend, Renee Zamora in a recent blog post cited by RootsMagic.com in their newsletter. Her post is called "Do I Still Need a Desktop Genealogy Program or is FamilySearch Family Tree Enough?" RootsMagic.com describes her article as follows:
While at conferences, we often get asked why a person needs a desktop genealogy program (like RootsMagic), when there are websites that let you build trees. The following is an article written by Renee Zamora (one of our great tech support agents) for her personal blog. The article was specifically written using FamilySearch Family Tree as an example, but can easily apply to other sites which let you put your tree online. Renee has given us permission to reprint the article here in RootsMagic News, and also to create a PDF version of the article that you can share with others.The original article is in Renee's Genealogy Blog.
Renee gives a huge list of reasons for maintaining a local genealogy program. If you have this issue come up and need to give some reasons, I think she about covers it all.