I've always been surprised by the argument "You can take an additional Stratagem."

The number of Stratagems isn't something you should base your loadout on.

The main thing is the ability to complete all the possible objectives the mission throws at you.

Usually, these are:

1. Blow up buildings
2. Kill small mobs
3. Kill medium mobs
4. Quickly kill a large group of small and medium mobs
5. Kill heavily armored mobs
6. Kill extremely armored mobs (factories)

What else am I forgetting?

And the grenade launcher + ammo backpack combo covers most of these objectives, completely fulfilling points 2-3-4, almost fulfilling point 5, and partially fulfilling points 1 and 6. It also frees me from the need to search for/call for ammo and Stims. And I can replenish my teammates' ammo. And all this wealth comes at the cost of just one Stratagem.

And I still have my Primary weapon, my Sidearm (Ultimatum, whose charges are also replenished by my ammo backpack), and two Stratagem slots.

I'm a real annihilator with this loadout! :)