If done right, and your business is in good shape, you generally don't need to worry about other extensions.
It does block out would be "domain squatters" or people trying to redirect to their own company, but a good Copyright Lawyer and a Registered Name will take care of that!
This is why you don't see AT&T buying "verizon.biz" and directing it to them. They're competing in the same space and it's a direct violation of fair business practices and copyright. Easily take that domain name in court.
Now, if you buy apple.us, and you sell microwaves, it's not a competing brand, so you are legally able to use that domain name without much recourse from the parent company.
Worth it to buy extensions? Not in my opinion. Market your own brand, trademark it, or an established business, and you have nothing to worry about.
*your results may vary