benefits of a fabric one is weight. a metal one full to the max is pretty heavy. makes a big difference by the end of the day and your tired and it is 100 degrees outside thats when you say i should have bought the fabric or aluminum.
they are lighter so dumping a full bag doesn't weight as much and you can brush up against small bushes and what not and not tear them up.
when i had my exmark tthp it had the bagger kit and when passing close to small bushes or ornamental trees the edge of the bag could brush up against them without doing any damage or trying to rip them out of the ground.
i have a metal grass catcher i use for my 36" metro and now my toro grandstand and i once brushed up against and ornamental tree and it started to rip it out of the ground.
another reason would be a fabric one usually is produced by the manufacturer of your mower which would also come with a bracket that hooks on the deck which creates a better a seal than any universal metal grass catcher.
We have the accelerator aluminum catchers and love them. Like dwost said, being that they are aluminum they are very light. When these are full of wet grass they feel the same as any nonmetal catcher. We have had 2 of ours for about 5 years now. One has a little dent in it about the size of a golf ball (I say its that size b/c one of my guys ran over one and shot it into the catcher). Other than that, they don't tear or wear out, and other than the dirt on them they look new. Also I have never had one snag or dig into a tree or shrub. It seems like you would have to almost ram the catcher into one to do something like that.