Joined: June 2004
Firstly I must point out that accurately characterising your sample is NOT making an additional assumption. If the only black voters available to you happen to be American would it be an "assumption" that your sample of black voters consisted only of Americans ? Would it somehow become an reliable sample of black voters worldwide if you simply refused to "assume" or even mention that they were indeed American ?
Rather in claiming that your sample of intelligent designers is automatically representative you are making the assumption that all intelligent designers are - in this respect - like humans. Which might hold for proposed extraterrestrials, but would hardly be safely applied to the Christian God who many (probably most) ID supporters believe to have been the "designer" of life.
Stylistic and aesthestic considerations are good examples of similarities that are associated with designers rather than any other mechanism. I think that the relevance should be obvious. If you really want to argue that similarities are evidence of design you should look for the similarities that are best explained by design rather than those best explained by rival hypotheses.
And this is the point. Keeping the argument vague and superficial may "help" your position in a rhetorical way since it puts design on the same level as common descent. Yet unless a design hypotheses can lead to deeper and more detailed predictions - as common descent does - then there is no real equality.