For one, slime molds are marvelous examples of the importance of dispersal. When the going gets rough, organisms have highly ingenious methods for relocating. More importantly, slime molds are a great model for studying the evolution of cooperation. In the multicellular form, the cells in the stalk sacrifice their lives to ensure the survival of the group. Only those cells in the fruiting body will leave offspring. This altruistic behavior usually can only occur via kin selection, and as such, slime molds provide scientists a great system to study how social cheating is prevented and evolutionary conflict is resolved.
In addition to evolutionary biology, slime molds are great models for studying cell motility, signal transduction, cell-type differentiation and developmental processes.
Much of the scientific success of this model organism can be attributed to John Tyler Bonner. Before Bonner's work, slime molds were mostly a biological curiosity. Bonner was casting about for a PhD thesis at Harvard when he decided to test whether D. dictyostelium aggregation was enabled by chemotaxis or by other means (e.g. contact guidance). Using a clever approach, Bonner showed that aggregation occurred by chemotaxis, and the chemical attractant was a compound later given the name acrasin1. The photo below is taken from Bonner's paper.
Bonner wrote, with characteristic humility, "[The paper] is cited so often [because] I devised a physiological salt solution... which has been used by many workers in the field. It is a simple mixture of NaCI, KCI, and CaCl2, which keeps the amoebae happy and healthy.... I was greatly elated by the fact that they referred to it as 'Bonner's solution.' What could be more impressive than having a 'solution' bearing one's name? ... I occasionally find bottles in our laboratory refrigerator labeled 'BS;' it gives me a feeling that one of my graduate students is sending me a message."
Cool site alert: DictyBase
1. The term acrasin was descriptively named after Acrasia from Edmund Spenser's Faerie Queene, who seduced men against their will and then transformed them into beasts. Acrasia is itself a play on the Greek word akrasia, which describes the loss of free will.
Top Figure: Development of Dictyostelium (M. Grimson, R. Blanton, Texas Tech University)