John Wesley's classic statement of the objection goes like this:
But if this be so, then is all preaching vain? It is needless to them that are elected; for they, whether with preaching or without, will infallibly be saved. Therefore, the end of preaching -- to save should -- is void with regard to them; and it is useless to them that are not elected, for they cannot possibly be saved: They, whether with preaching or without, will infallibly be damned. The end of preaching is therefore void with regard to them likewise; so that in either case our preaching is vain, as you hearing is also vain.
The main thrust of my response to this objection is to argue that while Wesley's argument gets at something that seems to me correct, for in the sense that I don't think the counterfactual relationship between preaching and salvation quite works the same way as it might under Arminianism, but nevertheless the Calvinists have two motivations for evangelism intact: the obedience to command motivation, and the instrumental role motivation.
So I think I've actually done more to undercut Wesley's argument than I have to help it. Sorry, JW, gotta follow the argument where it leads.