Its not really an issue of trained vs untrained when it comes to simply opening the exit (removing the door, come on a monkey could do it)...its the training when things go tits up...when its dark, smokey, and scary...having someone with their wits about them is key to saving the day. Too many times the people in those exit rows are not the sort that will be able to respond in times of need..yes they probably can lift 35lbs but I doubt their instincts will be to do what it takes to get out and assist crew.
My guess is trained cabin crew would be an asset but so would better screened or profiled pax. The FF with the gold card is fine and deserves the better seat, as long as they take the responsibility very seriously. However, in this day and age of political correctness and the almighty dollar, those seats will remain open to those that pay the salaries more often, no matter their physical or mental abilities...Ive seen a FF sitting in the exit who couldnt even lift his own laptop bag into the bin (weak, small fellow), yet demanded the exit row every trip as he was ...platnium (I know as he was my work colleague)...and really didnt even need the extra leg room, it was more of a prestige thing for him..for me it has always been about being 'willing and able' to assist...