I don't open Internet Explorer, but I think its code maybe is used in other Windows features, such as desktop gadgets. Will I benefit from upgrading IE8 to IE9 even if I don't use it? Will Windows 7 become more secure?
Internet Explorer exposes a lot of its functionality to everything else via COM components and DLLs. Anything in Windows that renders HTML for example, uses
Any security patches/bugfixes for IE will thus benefit a number of things not related to IE. Later versions of IE are more secure than earlier ones.