I think "magnum" has become a buzz word that has to be hung on any new cartridge if its expected to sell. Lack of knowledge is one reason. The mis-information and rank exaggerations that were used to sell Weatherby rifles are still floating around. Also, if someone is buying their first rifle or their first rifle to hunt west of the big river, they have heard so many tales of the bullet proof deer, antelope, and bear as well as the vast distances one is expected to shoot, that they figure they'd better get something that is up to the task. Especially when they've heard that there is a savage, man-eating grizzly lurking behind every bush. And so the hype about the magnums kicks in.
That said, I'd rather face one of those savage, man-eating grizzlys with a .338 in my hands than a .270. When we cite Jack OConnor and his iconic .270, lets not forget he had a guide at hand armed with a rifle of what calibre we don't know. I daresay however, when Elaine shot her elephant with a 30-06, her PH wasn't armed with a .270. 
Very good answer, there is good & bad with these magnums. Those who want to think the 270/7mauser is the thing will quote O'Connor, those who like big bores will quote Elmer Keith, so which one was right? Well, both were right part of the time, depending on the situation. Mr. Keith wanted to be able to shoot big game at whatever angle he could & O'Conner being quite wealthy could stay out there until he picked his shot &
we all know that if you can take these perfect double lung shots only you can use less gun. If you want to be able to shoot an elk in the point of the shoulder & take out the lungs & do so at any distance that you are TOTALLY capable of doing so, now we are out of the 270 realm in my view.
So, a better question may be how much time do you have. When I go on a Western hunt I have a week to 10 days max & then duty calls. Mr.
O'Conner could stay until he got that textbook shot, whether it was a week or a month. How about you? The same thing applies to all of our
hunting weapons. I hunt with a bow, handgun, MZ, & rifles & they all work, & I could fill all of my AR. Deer tags with a bow, but it would take a lot
more time & I would have to pass up a lot of animals out of range, I think you get my point.
Another factor to consider is O'Conner was smart enough to know that if he made one item his trademark he would become famous. His item & his
crusade was the 270. Other younger writers observed this & did the same, for example we know Jon Sundra as Mr. 7mm & that became his claim to fame. Seyfried became the premium bullet man & on & on. O'Conner would distort the facts to promote the 270 & that is unfortunate. For example, he would compare the trajectory of the 270 & the 7RM, with the 270 he would quote his classic 130 gr. spitzer reload & for the 7RM he
would quote the Rem. Corelokt load which was loaded by Rem at a lower pressure & also the Corelokt has a blunt tip & a low BC. Then he would say,
see, the 270 is almost as flat as the 7RM, oh my!!!! The 270 is a trully great round & can stand on it's own merits but he made it a little bigger & it
worked, look at the difference in the 270 sales compared to the 280, many feel the 280 is better but it will never catch up, so Jack was a 270 Salesman. I was a college student & owned a 7RM back when he was writing this & I used it for Groundhogs to Deer & felt this was all quite amusing.
Of course he promoted the 30-06 & 7mm Ma. some but it was not his focus. I remember once he said that if the 338WM killed better than a 30-06,
he failed to see it. Oh really!! 'Tell you what. Sometime stand beside a trophy Brown Bear or Polar Bear mount & ask yourself, if I was hunting this
animal & he had me spotted & was facing me, which would I rather have, a 30-06 or a 338, give me a break!! Don't get me wrong, I liked alot of his stuff & learned from him, I guess what to expect from lung shooting Deer was one of the first things I learned from him. But he had a ton of
bias, & for that reason I would weigh what he said with the experiences of others & his input would only be part of a decision instead of taking it at face value.
Another very important fact to consider is other than the Nos. Part. starting to make a dent then you only had std. bullets which meant that mags
did not do as well as they should at close range if they were loaded to take advantage of long range shots, in other words too explosive up close.
If you gun did not like the Part. (& many did not back then because they were not as precise as now) then you did not have a good magnum bullet. Now the Part is accurate plus you have many, many bullets that can handle the stress. So, unlike the past, you can have your cake & eat it,
you are good to go up close & at a little more range than the std. rounds, if where you hunt does not call for that, then use a std. round & a std. bullet if they fit your needs.
There are folks that buy a magnum just because someone else did just like people buy an expensive car because their neighbor did. That applies to all material things & will never end. so what? Goofy people abound!! Sometimes it is ego, same deal, & not my problem.
Personally I don't need the extra power or range very often & therefore I don't use mags. alot, but on rare occassions I do. I base those decisions on the situations that I may encounter & not on what some so called Guru thinks. In 3 of my last 4 trips to Wy. I used a std. round. I my be going on a short Elk hunt this fall (cows) in CO. & in talking about the conditions I may take my 300 (in this case distance more than power).
What it boils down to is I use what I need to get the job done & it is up to us to know what we need. If a person feels they don't need a mag., I
am OK with that, the same should apply to those who wants to use a mag., if they can use it effectively, but non-mag. shooters are usually not OK with that for whatever reason, which does not effect my decision in the slightest.
This idea that someone wants to use a magnum to make up for poor shooting is an uneducated opinion at best. It would apply to as mentioned before those who buy because of someone else or ego but not experienced hunters who buy based on what works best for them.
It is a funny thing to me that most of the really good shots that I know use std. rounds & magnums as well. Why?
