To interface or not to interface

[Steven Schveighoffer]

A long discussion on the utility of interfaces has taken place on the  
announce newsgroup following my announcement of dcollections for D2.

Dcollections supports interfaces peripherally.  Meaning the types in  
dcollections are essentially concrete classes with interfaces tacked on  
for cases where interfaces make sense.

Whenever I've used dcollections (D1 included) I don't think I've ever used  
the interface for a container, only the concrete type.  But within  
dcollections, there's some functions that use the interface type to do  
(IMO) nifty things like compare two sets that have different  
implementations.

The general consensus is that interfaces for dcollections do not add any  
meaningful value, and that I should do everything with generic programming  
and templates.  My view is that interfaces are useful for binary  
compatibility and in reducing the footprint of executables.  But that  
doesn't mean much at the moment, because D is statically linked.

I'd ask the naysayers of interfaces for dcollections, and also the  
supporters: what is the point of having interfaces in D?  Are interfaces  
pretty much obsolete, and I am just nostalgic about their utility?

What do you think?

-Steve