Here is why User Account Control is so important:
Most operating systems have two or three different user modes. The highest level of which is known as "Super User". The super user can make any changes he/she wants to the system. When a person or program enters super user mode, it has access to every single setting and file in the machine. Without super user privileges, however, the program or person is subject to the security policies and permissions set forth by the system administrator and can be prevented from doing any serious damage. This is the very reason why Linux is so secure. In Linux, when a program or person wants to access a feature only available in super user mode, the system administrator is required to not only acknowledge the access, but enter his/her password as authentication for this feature.
When a virus attacks your computer, one of the most common types of attack is to enter super user mode and have its way with your computer. So, Microsoft decided to mimic the super user feature in Linux and develop UAC. UAC is also one of the biggest reasons why 7 is so much more secure than XP. In XP, any program running while an admin is logged in has super user access. This is why it was such a hotbed for viruses and other security vulnerabilites.
In Vista, UAC was introduced, but not refined. Users would often diable UAC because of the frequency of notifications, etc. However, in 7, this feature is much more refined and does a wonderful job of preventing malware infection in and of itself.
By disabling UAC, you are effectively stripping Windows 7 of at least 50% of it's security measures, if not more. You are allowing every single program or person to do anything they like to your system without your knowledge.
I myself have UAC set to the very highest setting on my desktop and have caught several applications/malware trying to modify things that they shouldn't be.