So my friend and I came to the same conclusion independently and I thought it was worth sharing here. Simply put, additional weapon slots aren't worth it, but there is a way to fix it.
Now, I don't mean to say that the slots aren't worth the energy cost, I mean that currently they aren't worth using at all even if they were free. I purchased one additional slot. My friend actually purchased both for a total of four slots. However, this past weekend we only carried two weapons each despite having the extra slots available.
Why? Simple.
Your first two weapons are pretty indispensable. Your primary weapon is, of course, your primary. It's your go to weapon in any situation where it's appropriate. But there will always be at least one monster type that your primary just isn't good for whether it's because of damage type, debuff applications, or speed. Being able to quickly switch to a secondary weapon that is more suitable is essential when you suddenly find yourself surrounded.
This makes your #2 weapon priceless. However, it also makes your #3 weapon worthless.
Your #3 weapon is something you only carry for special circumstances. Perhaps it's something you don't often encounter, maybe even a single boss fight. This is even more true for your fourth slot. Having to cycle through these weapons when you need to quickly go from your secondary to your primary is very annoying and can even get you killed.
Ideally there would be a way to bind weapons to individual keys, but of course 1-5 are already taken by the quickslots, so this may not be practical.
An easy solution (I think) would be to let players temporarily lock out weapon slots. This could be done on a drop down menu in the arsenal screen. When a slot is locked the weapon in that slot would not come up when you hit spacebar.
Example:
Player has weapon #3 equipped.
Player locks slot 3.
Weapon 3 is still equipped.
Player hits spacebar, weapon 1 is now equipped.
Player hits spacebar, weapon 2 is now equipped.
Player hits spacebar, weapon 1 is now equipped.
This would force players to still commit to a set of equipment the way they do now (no equipping different things on the fly) but by letting them lock out weapons on the fly, they could have things equipped that wouldn't get in the way until they are needed.
Thoughts?
Your reasoning behind how additional weapon slots are worthless is rather short sighted.
1. Weapons 1 and 2 are max level, but you want to work on a new weapon without sacrificing power. Suddenly, you're leveling it because you have a weapon slot.
2. Where you're going, you NEED a larger variety of weapons than just two. Bam! You have more than two suddenly. (see: firestorm citadel)
2.5. Like to have weapons exclusively strong against specific things, and be weak against nothing? Then you're gonna need more than just two weapon slots to carry an Avenger, a Faust, a Callahan and a Silversix.
If you're not using the mouse wheel to change weapons, too, then that would also grant the illusion that weapon slots are useless.