not to be a jerk, but i think it is a misinterpretation coming into play here. you could do a "normal" set of bench press, or even a few sets, and really pay attention to what forces you to end your set. i would bet that it would be your triceps, and not your chest.
for me, its definitely the triceps that end the set before my chest does. therefore, if your triceps are going to give out on say rep 10, you definitely wouldnt want to pre-fatigue them before a set of bench press, because then they might give out at rep 6 or something, further taking away from the activation of the chest in the bench, if for nothing else because you ended at 6 instead of 10. does that make sense?
in other words, if you really concentrate and find that its your triceps (or maybe the delts) that give out before the chest, then you have to assume something like this: you terminated the set after rep 10 because your triceps (and/or delts) simply couldnt push anymore. but, your chest could have had several more reps left before it would have been ready to give out. by doing a single joint/isolation movement for your chest, immediately before hopping onto the bench, you are taking away the advantage that your chest had over the tris and possibly delts. that way, when the tris and delts are fried and ready to end the set, your chest is also fried.