The pH of stomach acid is in the same range as the pH of lemon juice, so I don't see what you would gain by using lemon juice. Stomach acid is specifically designed to digest raw meat, whereas lemon on meat is a modern culinary invention. Why settle for less than the best, as nature intended?
For people with a weak digestive system, I think it can be beneficial. It predigests the meat and allows bacteria and molds to go to work on it right away. If we all lived as nature intended, it would likely not be necessary. But in this day and age, many people may be too damaged to fully absorb all the nutrients even in fresh raw meat. So whatever can aid in digestion may be a benefit.
But maybe you're right, and it's only detrimental. I know I personally do better when I take the time to marinate it than when I eat it alone. But that might be because when have it marinated, I also add plenty of raw eggs, which I usually don't do if it's not marinated. As the marinated meat absorbs the slimyness of the egg white more easily without becoming sticky and slimy itself. This part is purely a preference, as my problem with raw eggs is the slimyness of the white.
When I mix even a large number of raw eggs with marinated ground beef, it turns the whole concoction into some sort of cereal with milk, it's quite tasty. If I use less eggs, I can barely tell the eggs are there as far as texture, but the taste improves because of the fat in the yolk.
I need the fats from the organic free-range eggs because I can't get grassfed beef, so I need to have only the lean muscle meat, as the fats, organs and bone marrow -which are otherwise the most nutritious parts- contain all the stored toxins. But if I eat lean meats alone with no fats, I think my body has a bit of trouble dealing with all the proteins and with no fats. The same happens when I eat lean wild fish alone. I try to mix it with avocados otherwise (just goes better than with eggs, I think)
I had some raw organic pork ribs recently that were very fatty, and they were delicious and my body loved them without any marination. The same happens when I have raw organic free-range chicken, which is also fatty, particularly because I also have all of the bone marrow along with the skin and meat and I even eat some parts of the bones. But when I tried to have the raw fat from conventional feedlot finished beef, it was disgusting. Mainly due to the texture.
I still have unmarinated raw beef every now and then, when I can't be bothered to grind and marinate it. Usually if I have to eat at someone else's place. I wouldn't say it's a problem. But lately, a while after every meat meal, I feel euphoric and peaceful at the same time. That doesn't happen if I have only lean meats with no fats. And I'm more likely to feel a bit sluggish and annoyed instead, if only for a little while.