One other serious disadvantage of keyword indexing is that like items may not be together in the index because their titles do not contain the same words. When keyword indexes are used in the right circumstances they can be very effective. You don't always look things up by subject. You may be looking for the name of an association, for example. If you are looking for the Association of American Aardvark Aficionados, it would really help to have a keyword approach to Aardvark in case you don't know the full, correct name of the group.
Quotation dictionaries almost always use a keyword approach.
That lets you look up the quotation by the words that are actually used in it. This is actually indexing three quotes. Daylight Savings Time. Sometimes you will have to use an index which is very poorly constructed. One common problem with poor indexes is subject headings that are very broad with too many items under each one.