Opportunity knocks, but does not respond to need
Though arthropods display a wide range of specialized appendages, it's important to remember that these limbs did not evolve because arthropods "needed" them. Rather, natural selection favored the evolution of those limbs, just as it did for the evolution of the exoskeleton the traits increased the ancestral arthropod's fitness.
There is only one catch what you inherit has its limitations and sometimes even gets in the way. Let's take a look at some of the constraints that evolution has placed upon the arthropods.