In the admin client, you can see the sizes that the view indexes are taking for any particular database. It's a surefire indicator of whether a view is being used or not.
When I am refactoring an old notes database (client), one of my surreptitious improvements is deleting all the customer-facing views that are not being used.
My rule of thumb is that a well designed application really needs only 5-10 customer-facing views.
Also, a big performance boost is critically looking at those views which could be downgraded to a manual refresh instead of the automatic ones. Surprisingly, some customers completely accept the tradeoff between speed of display and up-to-datedness of their views. (depends on the application, of course, but good for the infrequently-updated-but-containing-loads-of-data-applications)
One feature which I sadly miss was the ability to select a document, press 'shift' - or was it 'ctrl', click on another view, and the document would still be selected in the other view.
I must also, guiltily, admit to sometimes creating 'sloppy views' used only by me when I can't be arsed to do a sort algorithm, especially when sorting over more than one column.