This feature has now entered the SharePoint sphere with SharePoint 2010. In theory I agree the feature is a good one, but what about security? While the web is all about the masses, and any single person will be anonymously lost in the crowd, this is not necessarily the case in the enterprise.
I created a small test where user A uploaded a document with the term “pertaining pepsi” and user B one with the term “pertaining dr. pepper”. Each document is only available to the user uploading it.
In SharePoint 2010 search terms will be added as suggestion when the term has been used 6 times and someone clicked a result. User A searched for “pertaining pepsi” several times and opened the result document. Then I logged in as user B, and started to type “pert”
And lo and behold, it suggested the term with “pepsi”, even though the user don’t have access to any documents with the word.
The proper way of doing this is to execute a search for each suggested term and check if you have any results before displaying it to the user. It will require more search power, but at least you won’t breach security when your secret project is suddenly exposed to the rest of the company just because the team working on it searched on it so much.