I think both arguments hold true for various reasons.
When I started benching I was pretty flat natural small arch, wide elbows. This was ok until I got up to ~90kg benches, i started to get sore shoulders, moving my elbows closer to my body fixed this. bench didnt go up any faster.
When I got heavier 95, 100, 105 kg I started to arch up more, my bench didnt , hasnt gone up any faster than it did earlier, infact I think I find it harder to increase my bench now., could just be a food not big enough thing but.
What I have noticed is that when you bench flatish, solid core, elbows in feet planted, you can do more, bigger sets higher reps etc. When I am arched up , going for 1, or 2 reps I do feel moore solid, no question. If I stay really locked arched I get a sore back, not injured just fatigued, and sometimes in seems cramped, for holding the lock for an extendended period.
I have also noticed that when I am not arched the weight will either go up or it wont.
If I am arched up, i find I am able to fight it more, and really push it, sounds good??? Well I also find that when i really push it I tend to twist, and when arched twisting is nasty, I have strained my back a few time now doing this, The last was when I videod that 120, i was tired and pushed it., didnt work out for a week or two.
I think an arch should be incorporated when going for 1rm or low sets. But after you have got some ground work done, you need to have a strong core, strong back, strong hammies.. to support a strong pectoral girdle of muscles, it is your anchor so you are solid and can focus on the push that is in your face. Most novice lifters should not really be attempting big 1Rm sets, and maybe more focus should be on getting strong, and getting used to pushing and controling the weight.
I would recomend for normal sessions of gym lifts that you focus on the exercise of working your pectoral girdle muscles, getting stronger and in control, and then worry about an big arch, powerlifter type stuff until you want to do powerlifting.
Also when watching those vids on u tube, be aware we are not all 18inched thick through the chest, we are not all powerlifters, were you only move the bar 6 inches. A normal persons elbows will come past your back, esp if you want the bar to touch your chest.
Just stay solid, have a good foundation, get in a comfortable position, dont wiggle or lift your ass, keep your feet planted, elbows at 45degrees from your body, and a slight natural arch.
Best of luck..