Well, if the aim is simply to keep others from using the same domain name with a different extension, there are a
lot of other extensions you'd need to register beyond just .net and .org.
Back In The Day when there were only a handful of domain extensions in widespread use, it sometimes made sense to register all the most common variations. It might still in some circumstances today.
In the end, it's pretty much up to you. Decide how much you're concerned over this versus how much it's going to cost you to register and maintain registration for these domains... and whether it's going to accomplish anything for you to register only the .net and .org and leave other extensions "open." (Or spend the fortune it would cost to try to buy up every different extension out there.)
On the other hand, if there's a solid business or technical reason for registering the different domains, by all means go ahead. By that I mean, if you have some use in mind for these domains and using a folder within your existing domain file structure or setting up a subdomain won't work for you for whatever reason, then register them, for sure.
Bottom line, the answer to your question is a firm and resounding "it depends."
What do you expect to achieve by registering these additional domains? Do you have any intended use for them (or is there at least the possibility of some future use)? How many domain variations do you plan to register? How much will it cost to register them all and maintain the registration over time? Do you anticipate getting enough return from that investment to make it worthwhile?
Answer these questions, and the answer to your original question will likely become more clear.
--Torka
Domain Extensions (com, Net & Org)
