Well, weed does affect your motor skills a lot and generally tends to relax people, but the fact that you've expreienced it that way in a party, sprawled on a big sofa talking to a big boobed slut doesn't mean the effects are the same in different occasions, ie in combat.
I read an article recently that the Zulu Impi (Berserkers, in a way) were the stone-age equivalent of storm troops thanks to the psychotropic substances they used before combat, along with the ritual dances the Shaman presided. You'd be shocked that the main thing used was high-THC weed, which shoots your stamina to the sky, making some people laugh, others talk and talk, in this case the ritual was supposed to focus the warriors in the combat to come, and it always worked... The Zulu raided a fort of 5,000 Englishmen armed with rifles with approx. 2000 impi and they slaughtered them.
Another substance used in combat by the Impi was another kind of plant, only to be used when the warrior was cerain to die, it made him go totally berserk and absolutely fearless (mind you if you're fearless and careless in combat you will most certainly die), I can't remember well what the substance was but I guess something like Khat (amphetamine-like effets, plant grows in western Africa and Central America)
mixed with halucinogenous funghi.
As said before, weed also augments your perception, it makes the brain hypersesitive to any kind of sensory stimulation, basically speaking you see, hear, taste, smell, touch things with a x2 modifier
That is, if the stuff is "good quality" (high THC and natural) and you're not under the direct influence of some stress or worry, hence the importance of the shaman's "initiation to battle" rite.
Of course, if you're a pretty nervous person, had something that shocked you emotionally bigtime recently
don't smoke it. Imagine what a pure, entire spliff of Amsterdam skunk could do to a guy with clustrophobia if you lock him in a bathroom and turn the lights off.