And, frankly, most games have WASD and about 4 other keys that you need. Let's be realistic. Even MW2 doesn't have enough keybinds that you NEED to use mouse buttons (though I do always try to put melee on one of the mouse buttons). I men, let's look at your list...
MW2 is hardly demanding in the key/button/function department, which is why i never mentioned it, i mentioned games like Stalker and Opperation Flashpoint, games that actually
do have a ton of buttons.
Just playing Stalker:CoP (which is not the most key heavy game i can think of if im honest, Ofp or Arma probably take the cake), i was very happy to have my Mouse 4 and 5 buttons, without thouse, i could not have had all the keys that i might need in a hurry within easy reach, i'd be out two keys that would have to be placed in less ideal locations (something that would require me moving my hand away from WASD), but since i had them, it was never a problem, and i had all the essentials at my fingertips.
And thats what it's about, i could still play Stalker:CoP without thouse 2 extra mouse buttons, yes, my keyboard has 108 keys so it's not like it would have been impossible to find a place for it all, but it let me have all essentials at my fingertips, nomatter what the situation, and that's quite an advantage, one i shoulden't have to explain..
But ok, lets just look at this game and what it has, and my keybinds for it (there's some duals in there, sepperated by "/"s, and atleast one key specific for the 108-key keyboard layout (the "½" key, it's just above TAB and infront of 1)):
Move forward. (W)
Move back. (S)
Move left. (A)
Move right. (D)
Lean left. (Q)
Lean right. (E)
Sprint. (L-Shift)
Jump. (Space)
Crouch. (L-Ctrl)
Low Crouch. (L-ALT)
Fire. (Mouse 1)
Aim. (Mouse 2)
Next weapon. (Mwheel up)
Previous weapon. (Mwheel down)
Reload. (R)
Use. (F)
Change ammotype. (Y)
Change firemode up. (Mouse 4)
Change firemode down. (unbound)
Activate grenadelauncher. (Mouse 5)
Bring up detector. (Z)
Stop bleeding. (F1 / C)
Use health. (F2 / X)
Use Antirad. (F3 / ½)
Use food item. (F4 / V)
Open inventory. (Mouse 3)
Open PDA. (P)
Weapon 1. (1)
Weapon 2. (2)
Weapon 3. (3)
Weapon 4. (4)
Weapon 5. (5)
Weapon 6. (6)
Use shoulderlight. (Capslock)
Use Nightvision. (TAB)
Quicksave. (F5)
Quickload. (F6)
Pause. (Pause)
Exit to menu. (Esc)
Now that's 37 buttons right there (ignoring all MP specific keys, am i forgetting any?), 36 of which you will need (you can do without "Change firemode down", just having one of the two will work).
Hopefully, the point should make itself by now.. yes, having 2 extra buttons on the mouse for all of this is very practical, you're not using your righthand thumb for anything otherwise, but if you've got them, that thumb can control 2 of thease functions, which is two more functions freed up that can then be placed within easy reach of your left hand "WASD zone" (yay i made up a word! i think..), infact only "Open PDA", "Exit to menu", "Pause", "Quickload" and "Quicksave" are out of my reach with this setup, and they are all unessential in combat, i wont be reaching for them unless i have the time.