And yes, I happen to think it would be a good idea to supplement Buth with a more traditional text book (I like Machen.) But Buth himself may have a different idea.
It all depends on what one wants to do. If one wants to learn to talk about Greek in English, then any number of traditional books will do fine, followed by more and more linguistically inclined works like Runge. However, be forewarned that that road is ultimately a dead end. "Twenty years of schoolin and they put you on the day shift". True fluency cannot be developed by 'grammar-translation'. For that, a different trajectory is necessary, which is why the Living Koine Greek materials were developed. They will actually start to develop a different web inside the brain.
As a PS: in Greek studies I think that there are several pedagogical errors that even go beyond 'grammar-translation'. A primary example is the presentation of 'contract verbs (verbs accented perispomena in 1s indic.)' in abstract, artificial forms that require conscious processing before the real word pops out. χρῶμαι 'I use (+dative)' is a real word. *χράομαι is not a real word, nor is converting it into χρῶμαι a process that fluent Koine Greek language users ever performed. Practicing that conversion for a hundred thousand reps is a guaranteed way to block fluency and to build a natural inclination to think outside of the language rather than from within the language. Please note, I am not saying that the etymological information cannot be explained and provided to the student. I'm a linguist, i actually enjoy notes like that. It makes an interesting sidebar. But the training in the language must take place with real words if the student wants to build a real language. So while I consider 'grammar-translation' a poor methodology on general pedagogical principles from Second Language Acquisition studies, in Greek 'grammar-translation' is even poorer. Its persistence in classical studies is a remarkable testimony to human inertia.