it assumes all life, from nanobes to bacteria and archae to plants, animals, fungi, and protists are too complex... not just animals.
I'm no I.D. proponent (*shudders*) but they would say that the human appendix is part of the immune system. What about all the other vestigial parts? they'd say that we don't know it all but they DO serve a function... we just haven't found out what it is.
as for the appendix, it is a vestigial structure for sure. As whether it's useless or not, here:
http://www.talkorigins.org/faqs/vestiges/appendix.html
"However, contrary to what one is apt to read in anti-evolutionary literature, there is currently no evidence demonstrating that the appendix, as a separate organ, has a specific immune function in humans (Judge and Lichtenstein 2001; Dasso et al. 2000; Williams and Myers 1994, pp. 5, 26-29). To date, all experimental studies of the function of an appendix (other than routine human appendectomies) have been exclusively in rabbits and, to a lesser extent, rodents. Currently it is unclear whether the lymphoid tissue in the human appendix performs any specialized function apart from the much larger amount of lymphatic tissue already distributed throughout the gut. Most importantly with regard to vestigiality, there is no evidence from any mammal suggesting that the hominoid vermiform appendix performs functions above and beyond those of the lymphoid-rich caeca of other primates and mammals that lack distinct appendixes. "
"Furthermore, from systematic analysis we know that the rabbit, rodent, and human appendixes are convergent as outgrowths and constrictions of the caecum (Shoshani and McKenna 1998). It is thus very questionable to conclude from these animal studies that the human appendix has the same function as the other non-primate appendixes. "
"Of course, over a century of medical evidence has firmly shown that the removal of the human appendix after infancy has no obvious ill effects (apart from surgical complications, Williams and Myers 1994). Earlier reports of an association between appendectomy and certain types of cancer were artifactual (Andersen and Isager 1978; Gledovic and Radovanovic 1991; Mellemkjaer et al. 1998). In fact, congenital absence of the appendix also appears to have no discernable effect. From investigative laparoscopies for suspected appendicitis, many people have been found who completely lack an appendix from birth, apparently without any physiological detriment" (took the annoying sources at the end out)
"In sum, an enormous amount of medical research has centered on the human appendix, but to date the specific function of the appendix, if any, is still unclear and controversial in human physiology"