I don't think anyone has ever advocated not reserving flights that you know you want to take - of course you should do that. The question is should you make the ticket open-dated or fix the dates on the ticket? Making a reservation for a specific flight is completely independent of whether there is a date on the corresponding ticket coupon. Historically, a good travel agent will be able to hold flights for quite a while even when you haven't had any ticket issued, although this is getting harder to do as airlines move towards exclusively electronic ticketing.

As I see it the primary argument for getting the ticket issued with specific flights is to protect you in the event of a route being withdrawn by OneWorld - in this case you would then be carried on alternative carriers, even if that breaks other xONEn rules.