Just one data point. I think that it would have been more difficult to fit my fenders if I had used the dumdum material. They were difficult enough to to line up without having to worry about trying to squeeze the sealer in our out, as you try and carefully line up the fender. Maybe if your car has never been in an accident, this would be easier. For me, with front and rear fender (and rear valance) damage, it was pretty difficult to get all lined up. Getting the beading in there would just be that much more challenging too. I can say that I don't notice any issues with getting water in between panels (though I don't drive in the rain) and I don't have any squeaks and rattles from that area - but plenty from under the dash and from that big flat vibrating bonnet....
Pat