2. Cross Platform Compatibility.
3. While not inherently more secure, it's currently a smaller target than MS Office, which lowers its overall threat profile.
4. Getting away from vendor lock in. OpenOffice and StarOffice are both backed by Sun Microsystems, but there are actually a lot of other flavours of it being put out by different developpers.
5. Extensibility - Like Firefox and Thunderbird and Linux and other open source packages, if you find there's "that one thing" you really need as an add on, odds are some geek somewhere has come out with an extension to do that.
6. Plays nice with PDF. In fact, plays nice with far more file formats than MS Office does in general.
7. To repeat #1 - Cost.