As a minimum we buy the three TLD, .com, .net and .org for a domain name.
With most registrars you can forward the emails and web traffic from the two you don’t use to your main site at no extra cost.
The value of the extra domains becomes apparent when you start adding e-Commerce or company blogs or test or administration sites. Whilst your main public-facing domain can be on e.g. the .com you can host test and development sites on the .net or .org.
With Open Mutual we have our .net as our main public facing web site, this .org as a “blog” and the .com is used for client hosting and is used for our business continuity testing.
Most hosting providers allow you to have multiple TLD (usually up to 10) on the same host (shared hosting reseller account or a V-server) as well as countless subsites so the cost of running multiple TLD is just the cost of the registry fees (say an extra USD 20 per annum) plus extra time in administration of these extra domains.
If the domains are on completely different infrastructure (different DNS and servers) then the administration site can be used to advertise service status for the live domain.
Done right, the administration costs are neutral because when you are planning your business continuity you can deploy your backups onto the other domain names and hosting accounts without interfering with your live system or, equally, you can install new versions of software on the other domains and test the impact by comparing the live and test systems.