Falcarius is so much more than arms and leaf shaped teeth. It is a rather long dinosaur that tells us a lot of information about the other members of its family both before and after it in addition to being a member of that very unique family. Falcarius has a long neck and a long tail counterbalancing that neck and its bipedal stance allows for a great reach with the head that is on the end of the neck. The arms of Falcarius were also unique in that they were robust and were capable of extending through a rather interesting range of motion. Though very much like their Maniraptora sisters, Therizinosaurs developed a different set of motions in their forelimbs. In fact, Zanno 2006 states that 20% of the synapomorphies of Maniraptora and Therizinosaur actually show that Therizinosaurs were reversing their pectoral girdle conditions and thus the range and types of motions they were capable of producing in their forelimbs. Twenty percent is a significant change from similar animals. Not requiring their forelimbs to grapple with prey and instead using them in a different manner entirely, such as reaching for vegetation, would certainly alter their range, and therefore their anatomical makeup, significantly over many generations. This, then, makes sense that Falcarius is little changed as a basal ancestor, but the fact that it still possesses many of those changes and represents transitional stages of those changes is significant.
L. E. Zanno. 2006. The pectoral girdle and forelimb of the primitive
therizinosauroid Falcarius utahensis (Theropoda, Maniraptora): analyzing
evolutionary trends within Therizinosauroidea. Journal of Vertebrate Paleontology 26(3):636-650