You're describing FFXI here, you know. In FFXI, exp gain is relative to enemy level. Kill an enemy that is the same level as you, and you'll always get 100 exp. Kill one that is 2 levels below you and you'll get 75 exp, kill one that is 10 levels below you and you'll get nothing at all, not even one single point. The enemy level range from 0 exp to max exp from a single kill increases as you level up. a lv99 character can get exp from an enemy that is 20 levels below them, while a lv15 wouldn't get anything from something that was level 6-7 or so. To add to that, even with relative exp gain which forces you to always fight higher and higher level enemies, the exp you need for each level also increases. At lv1, you need 500 exp to reach 2, but at 74 you need around 45000 exp to reach 75.
Oh, but it doesn't stop there. Each job has a skill ranking in various weapons and defensive skills (going from A+ being the best to F being the worst (well actually I think a rating that is even lower than F exists in the game, but they are very rare)), and you only get a skill-up in each of those skills by performing an action on an enemy that is within an appropriate level. You can have a level 1 crab swing at you for all eternity without gaining a single evasion skill up if your evasion skill maxed out for level 10. Likewise, you will never cap your greataxe skill at level 99 by hitting a level 75 enemy. You'll get up to 75, but then it'll slow down severely and eventually stop, the same goes for magic skills, parry, shield blocks and martial arts "guarding". To add to this, your skill level in whatever affects your accuracy, and this accuracy is combined with the dexterity and accuracy rating you get from your regular physical level, which means that even if your greataxe skill is equivalent to level 75 while you're level 99, you can kill level 90 trash enemies because you can make up for it with your raw physical might/nimbleness/whatever. However, once you meet a level 110 boss enemy, that lv75 greataxe skill is gonna cause you to miss 80% of the time, and that is only because hit rate is floored at 20% regardless of stats. Not only that, with physical weapons, your skill rating also affects your attack rating, which is put up against the enemy's defense rating to make a ratio where you have to be above a certain amount to actually be able to inflict damage on things.
Basically, a white mage who has seen an enormous amount of combat and maxed her F-ranked evasion rating at level 99, will evade things better than a level 99 ninja who just leeched exp and never got a single attack thrown towards her, even if the ninja job has an evasion skill rating of A-. If the ninja actually starts taking hits from a high level enemy though, she'll gain evasion skill really fast, and eventually get higher than the white mage could ever get. This of course doesn't take into account that the ninja job also has access to probably 50 times as much gear that boosts evasion than white mage has, but I'm sure you get the point.
With that out of the way, I think the exp systems where you always gain *something* even if it is incredibly little doesn't really imply that you are still learning anything combat wise from fighting that enemy, but it could be an abstraction of your stamina and patience still increasing from fighting for an extremely long time. Even if your muscles won't grow to hit any harder or faster against a certain enemy, your muscles will still grow in endurance. At first you might only be able to swing your sword efficiently for perhaps 10 minutes, but after practicing against the same easy enemies for half a year, you'll probably be able to keep it going for hours without getting exhausted.