You can use a large Heavy Bag for other things (ground and pound, improvised throw dummy, etc). About the only other thing that you have to keep in mind is that you have to consider whether or not you have the space to place/hang the bag. Someone who has a garage dedicated to workout equipment wouldn't be hurting for space if they have 13'X13' area to hang a 100lb bag. But if it's someone who only has an apartment stoop or a corner of a room, then a 100lb may be too large to accomodate.
Ideally you'll have enough space around the bag to able to hit it with a round kick from all sides. You also want to take into account sway on the bag, because a bag repeatedly swaying back and forth into a wall will crack drywall or create holes (I know from experience). Further, the swaying action of the bag and impact from your strikes will create a lot of noise (again, I know from experience). You can get a decent workout in with a 40lb or 50lb bag. Getting a bigger bag is nice, but if space down't allow it's a waste of cash.
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