The Gospel of Mark should be included in the New Testament first and foremost because its account of the life and ministry bears such a close resemblance to what is written about Jesus in the Gospels of Matthew and Luke. This is such an apparent similarity that some critics even go as far as to argue that the Gospel of Mark is an amended version of the Gospel of Matthew. However, the majority of scholars argue that it is significantly different in a number of ways and has its own value and importance. Either way, the common account that these three gospels share have led to them being called the synoptic gospels, with "synoptic" meaning "common" to point to the common account of Jesus that they all share. This is certainly one test that would indicate very strongly if this gospel should be included within the canon of scripture; if it contained teaching that was not supported elsewhere in other parts of scripture then this would be a clear indication that it should not be included.
Secondly, I think this gospel account should be included in the New Testament because of the central message that it brings about discipleship and what it means to follow God. Note how this is encapsulated in Mark 8: 34-35:
If any of you would come after me, he must deny himself and take up his cross and follow me. For whoever wants to save his life will lose it, but whoever loses his life for me and for the gospel will save it.
This, to me, is a beautiful, concise and succinct statement of the true logic of following Christ, and this in itself corresponds to biblical teaching on discipleship in so many different ways.
The gospel of Mark begins with an account of John the Baptist. In the early days of Christianity, there must have been many individuals who saw John as a messianic figure. This gospel begins by describing John's role as a supporting figure in the mission of Jesus and clarifies that John was indeed beheaded and was not resurrected as was Jesus.
Furthermore, this gospel serves as a second witness to many of the many miracles and teachings of Jesus that were documented in the gospel of Matthew.
Scholars have now determined that the gospel of Mark was written before the other gospels in the New Testament; consequently it is a valuable text to include in the biblical canon.