Without getting super into it, Physical mods are essentially a scam of a mod.

• They do not affect your base damage, so they don't also improve your elemental mods.

• The effective damage increase is only a fraction based on the baseline ratio: using Buzz Saw on Gram Prime as an example, a +120% slash on a weapon with 75% slash results in a total +90% damage.

• The real nail in the coffin is that the bleed status does not scale with slash damage, it only scales with base damage, base damage mods (like Pressure Point or Condition Overload), and double dips with faction mods (turning a normal +30% Bane into a 1.66x multiplier, and a primed Bane with +55% into a 2.4x multiplier), while also being affected by other damage multipliers (like crits, headshots, etc).

There are legitimate uses for physical mods, but for regular melee use, the opportunity cost of using them is high, when you could slot in other things to pump up your damage output higher. A regular Bane will almost reach the same damage output as Buzz Saw on Gram Prime (of course vs the specific enemy), while also increasing your Slash proc damage further.

Legitimate uses include bumping up a slash ratio to being above 50% for dismemberment (to produce more Desecrate targets); for certain weapons (eg Shaku is full Impact) and stat stick abilities (Landslide is full Impact) that only deal 100% physical damage to get an actually effective +120% damage buff; for certain stat stick abilities (eg Whipclaw) that don't benefit from something better like banes, and can fish for more slash procs with it.

from what I know slash mods don't increase the damage of slash procs. maybe that's why

If this is for a Heavy Attack build, you might find that weapons with forced slash procs on heavy attack feel a lot more consistent like Scythes/2-handed Nikanas. The other benefit of relying on forced slash procs is that you don't need to worry about status chance/slash weighting. (not all heavy attacks have forced slash)

If this is for a Quick Attack build, then you probably care about status chance and slash weighting because you will be hitting enemies multiple times, and not all stances/combos have a forced slash proc.

Heavy blades only seem to have a forced slash on a ground finisher, but its slash weighting is high enough that you probably don't need Jagged Edge anyways. (Carnis Mandible might be better for + status chance, or Weeping Wounds for 12x combo builds)

The last reason is that +Slash mods only increase the upfront damage on the initial hit but do not effect the value of the slash ticks. I think the only mods that effect the tick damage are base damage mods like Pressure Point/Condition Overload, Crit Chance + Damage Mods, and Banes.

Usually because unless you are trying to get a certain status proc, physical damage mods kinda suck. Since a slash mod will adjust according to the BASE SLASH damage of the weapon, while elemental mods damage increase is based on all of the base damage of the weapon.

In addition to what neooffs said, it looks like a fully upgraded Buzz Kill (120% slash) increases flat damage (ignoring procs) on Gram Prime as much as a fully upgraded (non primed) Fever Strike (90% toxin) for the same cost. So if you only have one flex slot, going toxin instead is 1) an extra status type for Condition Overload and 2) upgradable to a Primed Fever Strike (165% toxin) for more damage if you've got the mod and capacity. If you have more than one flex slot, I guess it depends on the goals of your build and the statuses you already have access to on your loadout, but other mods might still be more beneficial.

From my (maybe questionable) understanding, slash mods have some value when:

1. you're low on capacity and the base slash weighting is high enough, a Jagged Edge (90% slash) or Carnis Mandible (90/60) (or non-melee equivalents) adds more flat damage than a 60/60 or equivalent capacity 90% elemental mod. Again, I suspect YMMV depending on statuses in your loadout, but I might be wrong.
2. you're trying to mess with the IPS weighting for more chance of slash damage over puncture/impact, for Desecrate dismemberment.
3. you're trying to mess with general proc weighting for a better chance of slash procs over other procs (this is the area I'm fuzziest on, maybe someone else can explain if/when you'd want to do this).

because it already does slash procs , damage of slash procs is all that matters for that build and increasing slash damage does not increase slash proc damage , just the likelihood of applying a slash proc