Someone who claims that faith is a good thing should not also use it as an accusation of impropriety.

I get the impression that that argument is used more to undermine claims that darwinism is a science than anything else.

Physics is a clear science; you can use the right equations and predict the motion of the Earth about the Sun, or the time a barometer will take to fall from a given height. This gives it a certain degree of credibility. The theory of evolution (and how the creationists love to remind everyone of that word, 'theory'!) is also science; but they would deny it, on the basis that accepting it suggests that it is as credible as physics or mathematics. If they insist that darwinism is a religion, then both alternatives start from the same basis of credibility; the creationists can then point out, quite accurately, that their version is older and has been around for longer, and therefore at least claim seniority.

There's a short story by Asimov that gives a very nice view of the whole argument.