Because they want to sell domains?
Never let it be said ;)
If you wish to use anything other than the .com, then buying other domains is quite important;
It stops cybersquatters, MFA and porn sites leeching your income and reputation.
However, if you buy (and use) the .com, your need is not so great, as you hold all the aces. Buying other domains is not so important for income and reputation protection, but is a good way to keep your options open for the future (eg .net and selected country domains).
NEVER bother with .biz or .info, if you have the .com - you'll never need them (not until the sun burns out, anyway), and it'll keep a spammer off the streets for ten minutes.
Protecting copyright and trademarks is important, but may be considered separately - the dates you set up are a matter of record, so will not be an issue.
[edited by: Quadrille at 8:27 am (utc) on Aug. 6, 2006]