I myself wondered about this parable/illustration till I found a commentary on these two verses from Mark 4 in Dr. J. Vernon McGee's Thru the Bible commentary series of books on the Bible. His comments really make sense when we think about how "religion" in the largest sense, has gotten away from the simplicity and the purity with which Christ taught, and instead metastasized into a huge aberrant all encompassing umbrella imbracing man made tradition's, social reform, and the very encumbrance's Christ spoke about of the Pharasee's. Here are Dr. McGee's remark's:
"Mustard is not food, it is a condiment. And the growth of a ustard seed into a tree is unnatural. This picturs the outward growth of Christendom into great organizations big churches, large programs, all produced by human energy and not by the Holy Spirit. The birds in the branches are not even good. They represent Satan". I hope this helps, G. Weldon
P. S. His website for listening to his Thru the Bible series is located at:
"http://www.ttb.org" and if your interested in my Biblical poetry mixed with my secular, my website is: "http://home.earthlink.net/~weldon" .