When you all buy a domain name – Made-up Example: cpwebhosting.net
Do you also buy cpwebhosting.com, cpwebhosting.us, cpwebhosting.info, cpwebhosting.org etc to park? To protect others for using your idea
If you have an ambitious project in the works, then yes, register all related domain names.
Normally having a multiple versions also helps in search engine efficiently.
If you have the money, you can buy up all the tld. No one can blackmail and extort you for money holding any of your domain name.
I think its best to buy up as many versions as you can afford. But only market one brand, and like you said park the rest at your main site. If you run a webhost, or use private name servers, you could use one(I usually use a .net) for the name servers, and still park it on the main site. Many uses for variants of a name.