Some people want the latest version of the software because it is better than previous version. However, not everyone is comfortable but if you have a proper backup, you will have confidence installing Service Pack 2 and start enjoying the improvements on Vista. Also, the Service Pack 2 installer can restore you to previous state if there are problems with the installation. This is similar method that Microsoft has done when they released Service Pack 1.
Other people will install only the critical and important security updates from Microsoft instead of installing Service Pack 2 for Vista. Then they will manually install the hotfixes by downloading it individually. The problem in doing this is your system is still missing the required service pack that future updates might or might not need.
Also, you should know that the service packs for any Microsoft products has been reviewed carefully to make sure it has the latest code base. You don’t want older codes that can cause instability and data loss.