I completely agree that for our packages it's less of an issue with size than others such as Symfony but I'd also consider it based on experience I've had in other areas good practice (e.g. thinking about using the symfony interfaces for session in the CMS or some of the guzzle interfaces in the http framework class) where I'd like to consider using the interfaces of these big packages so that other people can tap in and make alternative implementations that can just plug and play, but I just don't want a class sitting there doing nothing when I'm trying to use the interfaces. So I'd argue that it's just good practice anyhow to show as an example to others than as much of something that we HAVE to do.
And ya know that's a lot of why we have the interfaces, so people can plug and play. You don't need to have to use the entire stack interfaces. Again look at what Amy did - she just used a different name altogether. So you'd have
\JoomlaApi\Http\Transport interface
which again is more inline with me creating my own http component to plug and play than downloading all the interfaces when all I want is the http specific stuff.
Kind Regards,
George