Your point is quite right, I don't think it's self serving at all. Complex physiological diseases like cancer can be observed as tumours grafted to a scaffold in vitro, but you cannot observe the effect of drugs, it's relationship to the whole organism, without using a whole organism.
Monkeys have immune responses that are as similar to humans as we can hope for. Further, Mice while they have some conservation of immune related genes there are many divergent expression of immune related genes including alterations in master-gene (cis) and innate immune gene (CD4) expression [1] which may explain why they don't translate very well into humans. We have an awful time trying to get decent responses to allergic stimuli with mice and guinea pigs! Rodents have very robust immune systems in my experience.
http://www.ncbi.nlm.nih.gov/pubmed/15509386
http://www.ncbi.nlm.nih.gov/pubmed/18410164
http://www.researchgate.net/publication/11902612_Effects_of_...
I personally am about to embark on a scientific project that ultimately leads to animal experimentation. It's the development of a chemotherapeutic. Perhaps it's a self-serving thing to say, but I think that's one of the few places where animal experimentation makes sense; even the next-generation anticancer drugs, which recruit the immune system, don't make sense to be tested in mice, because in order to transplant the cancer cells, you have to start with immune-system-less mice (otherwise the cancer gets depleted by the mouse's own immune system).