The most common reasoning I hear for using a more powerful cartridge is that it has more 'knockdown power' in case you make a marginal shot. Makes me wonder if that translates to "my marksmanship skills aren't very good but I can make up for the shortcoming with a 300 RUM or 30-378 and then go on the internet and chide all of the less informed people on how bad of a choice their .308 or 260 rem is for anything bigger than squirrels"[:-]

My attitude is that a .243 through the heart is better than any magnum in the guts.