Usability trades off against features. Forces within Google are no doubt constantly pushing to add features to the search page. The temptation is great for two reasons:
- Since the page is so popular, you can guarantee traffic for whatever link or feature you add to it. When Google releases a new product, think of how tempting it must be to add a blurb to the main search page urging visitors to check out the new product.
- Google could implement more flexible and customizable searches if it included some options on the main search page.
For this reason, it's helpful to define usability metrics up front. The metric might limit the amount of time it takes for a typical user to accomplish the most common and important tasks. Or it might measure the likelihood that a novice user will perform these tasks successfully when visiting the page. Each new feature Google adds to the page will tend to increase the amount of time and decrease the likelihood that a visitor will perform the most common and important searches successfully.