I don't think there's a single person at Platinum games that would be able to actually answer why they thought forcing you to stop moving to bring up the sub-weapon menu was a good idea.
It's probably a balance decision, other games have done this before. Like in DMC1 you couldn't change weapons from the menu until you were grounded. Or it's more likely a RAM limitation, which is why not every action game has on the fly weapon swap.
I simply cannot find any fun in the gameplay of MGR. The camera is one of the worst I've experienced in a long time combined with the very strange parry system. I'm up to the helipad fight on top of skyscraper and I simply cannot figure out how it wants me to aim the sword.
Reading through this thread I feel like one of those people that didn't "get" Dark Souls. I can only assume I'm doing something horribly wrong with all the love the game is getting, but I just cannot get it to work.
Also, the controls outside of sword-based combat are so jank I can't imagine how they could be useful (such as grenade throwing and ninja running).
The camera is pretty bad but the parry system is fairly straight forward.
You parry in the direction of the opponent. The first step is to learn how to block and how to block multiple attacks. You can literally just mash light attack plus keep flicking the stick towards the opponent to force yourself to block. You won't get perfect parries but you will at least block stuff. Once you get a hang of blocking then you move on to perfect parry which is timed with the opponent's attack (when it's about to hit you although for some attack its sooner and some attacks can't be perfect parried).
Don't bother with the sub weapon system except for very specific areas. Try to fight everything with the basic sword as much as you can without using any items (even healing items). That's really the only way it will click. Try to perfect parry every red flash attack and dodge every yellow attack.
Your basic goal for this game is the following:
*Disable the opponent so that you have a Zandatsu opportunity. This is usually possible with a perfect parry or properly dismantling an opponent.
*Perfect parry as much as you can. Block every red attack. If you feel like you cannot block something because your camera orientation isn't correct, go for a dodge always. Yellow attacks have to be dodged, just time it so that you use the invulnerability frames.
*Learn to do Ninja run into slide into immediately Blade mode to quickly take out small fry opponents who like to sit back and shoot rockets/bullets. They are #1 priority targets in a group as are flying targets. You basically work up the threat scale... you try to kill the easy targets as fast as possible and then save the toughest guy for last.
*You need to learn the signs for when you can do Blade mode into Zandatsu opportunity. Usually there's a slow down or a haze/blur on the screen which is your cue. Common Blade mode cues are after a Palm strike, the 4th hit of an air combo or the slide. Starting out your way to play the game shouldn't be "how should I kill this opponent" but rather "how should I rip out his spine".
*Ninja Run to dodge small projectiles and to make space or gain better position. Don't be afraid to jump out of trouble either, you can Parry in mid air and in some situations that is the best option because enemies like Gekko are hard to hit with the parry on the ground (aerial parry has a bit more range).
*Use Blade mode to get out of bad situations or high recovery moves. Just tap the Blade mode button after a high recovery move. Usually you throw out a slash and it lingers out, you aren't sure if the parry will come out so it's better to go into Blade mode very briefly to sort of reset yourself and then go from there.
*When you have dealt with the long ranged projectile enemies, the rest of the fight is keeping track of enemy attacks and holding your ground while maintaining best camera viewpoint. Positioning is key, if you get cornered you are done in this game. Don't go for wild swings unless you are sure they will hit (most enemies block, some can even parry you).