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The author makes a very valid point; no-one who searches Medline (or any other similar database for that matter)in the hope of finding as much relevant material as possible should ever depend on a single means of so doing.
For instance, indexers frequently omit to apply the age tag "child" when indexing papers from paediatric journals; relying on this alone as a limiter would invariably miss many useful references.
In these cases, the AF (Any Field) search comes into its own. For instance, searching for child*4 or adolesc* or pediatric*1 or paediatric*1.af will find many more papers than simply using an age limiter.