I think you are right that it is wrong to say “[a]dvocacy of the proper political philosophy can proceed only from the proper objective moral foundation.” However, what one might want or expect from such advocacy likely depends on the philosophy of one’s allies.

There is a good argument that join up with, for example, utilitarians or pragmatists than the results are going to be muddled. If victories are won “purely” then the principles on which the win was based will further the objectivist philosophy and their causes.

To steal an example from someone, I cannot remember from whom, it is like forming an alliance with everyone’s favorite utilitarian, Judge Posner. Sure, at times our goals happen to align, but do I really want our joint victories to be secured in part by a person who thinks “justice” is a useless term? Any such victory would be prone to fail for lack of a proper foundation and/or may serve to further his viewpoint.

Of course I was just arguing the other day that it is counter-productive for my libertarian/objectivist father to decide to stop sending money to a libertarian organization because of what he thought was their pointedly non-objectivist, treatment of Ms. Rand’s 100th birthday this past winter. So, I don’t think that the refusal to ‘make common cause with’ libertarians is a good idea, I just that there is a better argument for it than the one you refuted above. Of course, I am only a self-described, “quasi-objectivist.” (imagine how long my comment would be if I were a true believer).