I have always found it a little annoying to have to open my inventory just to haul out a grenade or a flare to throw. What I find most annoying about the process is that I have to go into my inventory twice. Once to pull out the grenade and a second time to reequip the weapon. Would it be possible to have the option on the GUI of slinging the weapon? It would make two handed weapons single handed, but unable to be fired. This would mean that you would only have to go into your inventory once to get what you need. After you use the grenade that you pull out of your inventory you could unsling the weapon enabling use again. This would mean that one would only need to access the inventory once and the user wouldn't have to spend time finding a place to put his weapon in the meantime.
If this was combined with quick draw grenades on the GUI the only thing one would need to go get in the inventory for in combat is reloads, medic kits, and sidearms.