I totally disagree. It's not only a good idea, it's essential for certain core functionality to work at all. Take serialization, for example: without this feature it's impossible to write a good serializer as a library without code generation (messy and brittle) or reflection (very slow). To give another example, try writing a linear algebra library that's generic over the data type without this feature (and such libraries are essential for high performance graphics programming). The only way I can think of to make it work is to make callers explictly box their numbers into wrapper types you create, which is really ugly and causes various other problems.

I don't understand the idea that not having this feature somehow helps enforce dependency order either. Extension implementations of traits don't give you any more abstraction-breaking power than downcasting interface{} to a concrete type not defined in your package does. In fact, they're pretty much strictly less powerful.