Son sólo para apps triviales.
te mando este extracto de Liz Woodhouse:
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Autoincrement fields: These fields are good when the
sole purpose is to have a unique value. If the value will be used to link tables, if it will have meaning in any way (such as a customer number), if the value
must remain the same, don't use an Autoincrement field. Why? Well, there are some problems which make these fields risky: they can 'lose track' of which number they're on; moving data into a copy of the table (for rebuilding the table, or archiving, for example) can reset the field values if you don't do it just right (just right involves restructuring the tables before and after copying or moving data); repairing the table under certain versions of the BDE can lead to the numbers being reset. So, while it's possible to use these fields without ever having a problem, the risk is high and you should be aware of that beforehand.
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Tienes mucho material sobre este tema en
www.thedbcommunity.com
Saludos, Carlos.