You should only add toolbars that you will use. Having too many commercial toolbars causes problems.
When you are installing programs you will often see where they want to add a toolbar such as Google, Yahoo, AVG, and others. Usually it is chosen by default. I would recommend you not install the toolbar unless you want to use it specifically. Toolbar hopping can be not a good thing.
Why would you want to add toolbars? They have the advantage of adding some features and shortcuts to products of that company or ones they endorse. The Google and Yahoo toolbars make their search engines available easier (if you know address for a site use the address bar to type it in and not the search box as you will be relying on the search tool finding link and then you still need to choose. Some sites you are going to may not be indexed by a search engine). One of the big features people added toolbars for a whole was their pop-up blockers.
Why would you not want to add toolbars. They will take another row off your browser reducing the space (call real estate) available for the web pages to display causing you to have to scroll down more. They will conflict with each other. If you ever need to stop pop-up blocking this becomes a problem. There are times you want to allow pop-ups. Current browsers such as IE and Firefox include pop-up blockers in them. If you want to stop blocking pop-ups you will need to turn that off in the browser and each toolbar.
My recommendation is to only use one toolbar that is not part of the browser or none.
Thanks to Charles for the question suggestion.