Doesn't Cloud have like superhuman strength or something?
Anyway, in the off chance you could actually find someone to effectively wield that monster with agility comparable to a normal soldier wielding a normal sword, there are some significant advantages to that much bulk.
First, because of its mass, it would be effective against a heavily armoured target. Remember that impact force per area is squared if the impacting area is halved, and force is dependent on mass. Having all that mass behind the cutting edge could make it much easier to cut through armour or, failing that, would be effective as a bludgeoning instrument.
Amethyst Wind said:
The cutting part at the end was pretty demonstrative that it's bloody useless as an actual sword. None of the items were actually cut by the blade's edge, they were simply smashed by the descending weight of the sword itself.
It's essentially a club.
Pretty much. If you treat like a club with a cutting edge rather than a very heavy sword, the role of such a weapon could become clearer.
Second, as a fencer for my school, I can tell you that parrying even a 2 pound sword is surprisingly difficult. I would not want to be a soldier in the position of trying to block that monster, which from a video I saw once weighed in at like 60 pounds (I sadly cannot watch youtube videos on this computer so I can't watch the on you've linked). Blocking such a weapon would be very, very difficult.
Now, given that there are also guns in the FF7 world I don't really see the point of a sword, but if for whatever reason you felt like using a buster sword those are the advantages you'll have.