Agreed. The first time I read this it never never really made sense that the decoys are the 5 young volunteers and then Mundungus, who is clearly there against his will. On this reread though (I may or may not have cheated ahead and finished already, but it was only week ago so it's still quite fresh) it makes a bit more sense when we learn that it was Mundungus (via Snape's Imperius curse) who came up with the escape plan, or at least the multiple Potters idea. One would assume he was brought along as a way to insure that he this plan was and remained on the level. He is less likely to find some to way to sell them out if his ass is on the line with the rest of them.