Judging from photos and information about the races in which Merckx and others switched from downtube to bar-end shifters, it appears they favored bar-end shifters for races or stages with more sketchy pavement, cobbles and gravel, presumably to keep both hands on the bar.
I put bar-end shifters on my Univega hybrid with albatross bars a few years ago and it does help on sketchy terrain. I've considered bar-ends for my Centurion Ironman road bike for the same reason, since some of our group rides include mixed terrain. But I'll probably install a set of either MicroShift 7-speed brifters, or 8-speed Dura Ace brifters. Those are even better for keeping both hands on the bar on sketchy terrain.
I've heard claims that bar-ends have more slop in shifting compared with downtube, but I don't really notice much difference on my bikes. Might depend on the type of downtube shifter. I have Dura-Ace and Ultegra SIS bar-ends that offer indexed or friction shifting. I'm not sure I've ever tried Suntour barcons or other bar-ends.