MiCal is WAY quicker to access than iCal to check your schedule, which is what a lot of people do most frequently in iCal.
About memory usage, MiCal uses Java. Memory usage reported by Process Viewer or top reports all memory associated with an application, even that shared by other applications. In the case of Java applications, a lot of shared stuff gets reported. MiCal isn't really using all that memory. You can find Apple Tech Notes about this, particularly about Java and Virtual Memory, which reports HUGE numbers, erroneously.