MySQL Storage Engines - page 8
What is a Storage Engine?
As you may have been able to conclude from the above summary of the different storage engines available, there are few reasons not to use either the MyISAM or InnoDB engine types. MyISAM will do in most situations, but if you have a high number of updates or inserts compared to your searches and selects then you will get better performance out of the InnoDB engine. To get the best performance out of InnoDB you need to tweak the parameters for your server, otherwise there is no reason not to use it.
But if both MyISAM and InnoDB are so great, why even consider using the other engine types? Simply because they provide specific functionality that is not otherwise available. The MERGE engine is an exceedingly effective way of querying data from multiple, identically defined, tables. The MEMORY engine is the best way to perform a large number of complex queries on data that would be inefficient to search on a disk based engine. The CSV engine is a great way to export data that could be used in other applications. BDB is excellent for data that has a unique key that is frequently accessed.
Some of these are possible to do with InnoDB (our separate MyISAM logs, for example, could be combined into a single InnoDB table), but the flexibility to choose an engine type that suits you and the data you are working with is what differentiates MySQL from other solutions.