I would also quibble with this a bit. It's not the distribution of similarity that is evidence of evolution: it is the distribution of homologous (non-functional) similarities. Of course a mouse and a rat are going to share a lot of similarities in terms of anatomy, physiology, and behavior than if either one is compared to a nematode. This is because they have relatively similar niche.
It's the non-functional similarities that are interesting.
I can see where you are coming from here but I really don't think you can just dismiss anything functional, not to mention that fact that many IDists or creationists will dismiss the idea that there is anything non-functional to look at in the first place, see the many arguments based on vestigial organs for a reminder of this.
A whale and a mouse have quite distinct niches but they still share several clear and functional similarities which are a viable basis for considering them both placental mammals and would lead you to consider a marsupial mouse the outsider on the basis of such features. There are also a number of morphological feature on which we might come to the contrary conclusion, but the more features we look at and the more species we add to our analysis the more apparent it will be that there are more shared features between the placental mammals than marsupials, especially when we bring genetic data into the analysis.
None of this requires any prior assumption of homology, simply the systematic organisation of the organisms on the basis of similarities. Such comparisons will shoe up examples of both homology where gourps of similarity cluster together and convergent or analogous similarities which occur in disparate groups.
Maybe you are just using non-functional in an unusual way in terms of morphological features, it seems highly biased toward the sort of molecular comparisons you mentioned earlier, if not then your view seems pretty restrictive.
It isn't as if the lack of a clear slam dunk argument for every homologous structures against a creationist ploy should be the basis for the scientific definition of a term. When I discussed the pharyngeal arches with Randman he continually moved the goalposts back, the deeper the morphological, molecular and genetic homologies got the more evidence he required. There is no point ignoring perfectly good scientific evidence in favour of exactly the right flavour of evidence to convince those who are not open to conviction.
TTFN,
WK
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