I wouldn't go so far as to say it's nonsense. As you pointed out, good shot placement with a .243 is sufficient. The problem can arise when a new hunter and some not so new too, get buck fever and may not place the shot in the best spot, especially on a big Red stag like we get here in our local pine forests. Some of these stags can go over 170kgs dressed and you don't want them running 50m. A heavier cartridge such as the .308 30/06 etc with a suitable bullet gives a little more leeway as regards shot placement.
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