Thats not the arguement as your comparing two different builds. Of course I don't think those 10 stone soaking wet bicep boys should be partially moving weights far too advanced for them. I don't even want to get started one some of their training and pied routines.
However, if your doing partial (for the sake of this chat) reps with 50's, would YOU benefit more from doing 40's and taking them right down?? IMHO, yes. Depends if your ego can handle it though.
Just my opinion and you may find you grow more for doing the partials.. Everyone is different. Shoulders, for me, work best when burining them rather than pumping. That said I rarely train mine so my thoughts are null and void. lol