Was busy being sick and feeling miserable... And running dreams and nightmares but still.

But yes, with weapons you need to be holding them out to gain MSI. For example, cutters would only grant MSI when you hold the cutter out, however if you're wearing the kat claw set as well you're get a massive +4 MSI is you swap from a gun to a cutter.

Also catalyzer having MSI: low/med might work. Since the gun is about positioning MSI is helpful.

Now onto responses to Fehzor's post!
Bombs being less mobile with chaos won't really be noticed unless you're trying to be primarily a bomber in chaos (which is something I want to move away from). You don't really see the difference in MSI at low, and barely will see it at med - however if you are bothered by the MSD on chaos then chaos bombers should use shard bombs as they negate the negative. If you want to be using other bombs then going down a better class line would be good.

Don't want to go too crazy with the current boosts. From what I can see the kat sets are supposed to be balanced with the other class sets, but give a little of each to it. Giving MSI: low per piece will suit that, but again it will lock you into using the class weapons. They also give more specialized resist than other class sets do (often having more normal defense instead), so they work better in that area.

For the class armor buff I'd say give one set from each set that MSI bonus. I only applied it to shadowsun because I figured ASI sets shoot faster so it doesn't matter if they miss a shot every so often, whereas damage gunner sets typically need to land every hit (swiftstrike exists but I'm assuming we're using 5* shields here). For gunners ASI is more useful than damage, thus buffing a "standard" damage set is good.
On the same train of thought, damage for swordies is often more powerful than ASI. You're in everything's face, so if they flinch or die more then you'll be alive longer. Because of this vog cub would gain the MSI.
Lastly, bombs rely heavily on CTR. So we could give MSI to the bombtastic set to make it not a bad choice for bombers who don't like the idea of status vulnerability (alternatively keep it a mad bomber-only thing).

Defensive armors are fine and dandy right now. MSI is often a more offensive stat anyways, thus why plate mail sets have some sort of slowness to them (ASD for standard sets, MSD + ASD for ancient). You can still activate MSI abilities via using certain weapons.

Standard mercurial is fine as is. I don't want to give huge MSI bonuses across the board necessarily, but give more options to what has MSI. Mercurial is the shock/piercing resist armor, and it does both of those very well. It drops an additional status resist for MSI, which isn't a bad idea.
I will say that if ever we get special effects planted onto armors that mercurial be given a backfire barrier-esk MSI for 5 seconds when shocked(2.5 seconds per piece). Ho joy that would be amazing to see how fast you go with MSI: max (holding cutter/shard bomb) + speed boost + actual backfire barrier.
To completely counter what I said in the previous mini-paragraph, defensive sets could have MSI... But only if they follow this pattern. Give MSI: low and hack off a status resist.

Fallen set I think is fine, however some sort of weapon MSI might be okay to put onto it. Fallen doesn't need to be a powerhouse since it's got a fair amount of status resists.
However it might be kinda neat if it got the weapon-MSI increase at random, so sometimes it would be swords, sometimes guns, sometimes bombs. Makes for a bit of a... Fiendish situation.