I feel like people who don't like RE5's inventory system didn't take advantage of it very well or are just stuck on RE4's and unwilling to accept anything else.
I'll take real-time inventories over immersion-breaking ones any day, as fun as RE4's was. The only valid complaint against 5's is the amount of stuff you could carry at any one time, but the grid design makes sense since they gave us the convenience of shortcuts for the cardinal directions. It wouldn't make sense to have more than 9 blocks with such a design, otherwise it'd be a bit cumbersome.
With 5's inventory system being real-time, you gained the very welcome bonus of never, ever having to see another reload animation again by selecting the necessary ammo and just applying it to the gun and, bam--instant reload. One can get really damn good at it and it becomes like a fun game on its own to see how fast you can do it. Not only that, but you can do it during animations, which is huge: climbing up ladders, meleeing an enemy, dropping down from a vantage point, picking up an item (which also gives you i-frames), recovering from being hit by an enemy, and so on. It doesn't even have to be just ammo you mess with during those animations, either; you can rearrange on the fly or set up to discard something once your animation is over, mix herbs, etc.. You can't do any of that with RE4's inventory except equip a different gun or heal asap, or just rearrange stuff for the hell of it. Be honest, who hasn't spammed start in RE4 to pop a FAS or G+R+Y mix when taking a fatal blow, hoping to beat fate?
Another neat thing that comes from RE5's inventory system due to cardinal-direction shortcuts is the ability to quick-switch weapons, itself an invaluable ability in Mercenaries to save a combo. Say you have a handgun equipped and you see your combo flashing rapidly, indicating it's about to be lost. You don't want to risk wasting time trying to save the combo with x-amount of handgun shots and MAYBE saving it. What will most likely happen is that MAYBE you'll kill it, but only after losing the combo for taking too long, OR triggering a mutation that takes precious seconds to finish, also causing you to drop the combo. What you REALLY you want to do is aim that handgun, then hit your shortcut for, say, a magnum, and immediately shoot with it after hitting your shortcut.This causes you to skip the slower animation for normal weapon switching, and you'll immediately shoot wherever you were aiming the previous weapon before you've even fully drawn the new weapon. Combo saved. The key is aiming beforehand. For perspective, you can skip the rocket launcher scope with this method and always know where the rocket will land because you were already aiming there with the last weapon!
GOOD inventory management in RE5 takes just a bit of skill. Dexterity trumps memorization, but if you've got your item placement in the back of your head, even not-so-dexterous players can do the above, especially the ammo-to-gun moving since the cursor always starts in the middle when bringing up the inventory. Not to mention they can get even more time by doing it during animations.
The most skill RE4's inventory requires is organization prowess, which a lot of people lack anyway. I've seen some of your Steam screenshots, heathens. Rotation is a thing!
In case anyone's about to do Lord Saddler's bidding, I'm not knocking RE4's inventory. Shit's fun. I'm just saying I prefer the experience a real-time system provides; I like being a little more involved. Just like the attache case, 5's inventory is its own game. Pausing in the heat of battle to heal and stuff is not my preference, but it doesn't ruin the experience entirely, either.
Anyway, I'd just like to say that I appreciate all the little quirks RE games have where mechanics are concerned. I like them all--especially discovering them on my own or with the help of the community. It makes me appreciate the thought that went into them--or didn't!--and how they're so small yet significant. RE6 has some great ones, too.
Edit: I can't highlight to save my life on mobile, but thanks for the comment, Silky.