It's not so much about that specific case, any other side effect will get you in trouble. it's more about how 'extends' introduces 2 tricky features. 1) you get

It's not so much about that specific case, any other side effect will get you in trouble. it's more about how 'extends' introduces 2 tricky features. 1) you get a neat dispatch mechanism. if foo doesn't have a method check foo's superclass, and see what's available there, recursively until you run out of parents. 2) you can slip a different implementation in to some complex system, and they never need to know. A nice example might be a DatabaseConnection, and a TestDatabaseConnection, that doesn't actually talk to a database, but logs what was requested. The thing that sucks about inheritance is, you never really have a DatabaseConnection, you wind up having a MYSQLDatabaseConnection or a PGSQLDatabaseConnection, that take slightly different syntax. It's not real clear if you should make a TestDatabaseConnection for each database permutation, or just make a logging connection. Anyway, traits can fix this up a little nicer than interfaces and delegates can. Inheritance is helpful, but it kind of sucks because you only get to use it once, and it mixes two different concepts together. "easy" API extension by tacking on more features, and the cuckoo's egg that lets you slip different implementations in without a recompile. IMHO #2 is really what people are after, and you can get that by just using interfaces for everything.