Possible new entry for my book Thinking from A to Z
A particular kind of getting personal (or ad hominem move). Those who use this phrase typically do so to undermine the credibility of a speaker by pointing out the vested interest or highly motivated nature of their comments. During the Profumo trial Mandy Rice-Davies famously said of Lord Astor's denial of an affair with her 'He would say that wouldn't he', drawing attention to his motivation for denial. In that case it was certainly relevant and appropriate to make the remark, and it was devastatingly effective. However, in some other cases drawing attention to the speaker's motives may deflect attention away from any arguments or evidence that the speaker is actually using. These should be assessed independently of speakers' motives.
So, for example, someone who is both health-conscious and loves wine might cite scientific evidence to support the idea that drinking moderate amounts of red wine has beneficial effects. This might elicit the response 'you would say that wouldn't you', which would draw attention to the speaker's vested interest in discovering that wine drinking is compatible with a healthy lifestyle. Yet the speaker's motivation cannot affect the evidence: that stands or falls whether or not the speaker is motivated to cite it. As long as the speaker isn't misrepresenting the evidence (and possible counter-evidence or alternative explanations of this apparent health-giving effect), then the charge 'you would say that wouldnt you' does not touch the facts; it only reveals something about why the speaker might be so keen to cite those facts.
The phrase is best used against a speaker who, like Lord Astor, merely asserts a position which they are highly motivated to defend, rather than against those who use argument and evidence to support a position (which is also highly motivated). In the latter sort of case, whilst understanding motivations gives us a fuller picture, it should not cloud the issue and prevent us judging the arguments and evidence as presented.