Which makes a kind of instrumental sense, in that advocacy of this position aids the first group by innocently explaining away gender inequalities. (I think it's obvious that most people don't distinguish well, in political situations, between incidental aid and explicit support.)

Can't this be said of just about all straw men ? Yes, setting up a straw man may be instrumentally rational, but is it the kind of thing we should be applauding ?

Say we have two somewhat similar positions:

• Position A, which is false and maybe evil (in this case "we should discriminate against women when hiring scientists, because they aren't as likely to be very smart")
• Position B, which is maybe true (in this case ("the lack women female scientists could be due to the fact that they aren't as likely to be very smart")

A straw man is pretending that people arguing B are arguing A, or pretending that there's no difference between the two - which seems to be what P.Z. Myers is doing.

You're saying that position B gives support for position A, and, yes, it does. That can be a good reason to attack people who support position B (especially if you really don't like position A), but that holds even if position B is true.