My understanding of it is Binks quit as opposed to being fired. He wasn't happy with the direction the band wanted to go -- more straight forward rock -- i.e. less tempo changes and so forth. He basically left the band due to 'musical differences'. Also, as I recall, there was some issue as to how much Binks and Ian Hill meshed in terms of their rhythm section. At the time, they felt it would be better to bring someone a bit more rudimentary in terms of their approach to the drums -- Dave Holland. Holland certainly fit the bill in terms of his 'workman like style'.
So yeah, as I figured, Godshifter for the win. I guess I should have figured Binks left on his own. That guy was fucking great! I guess the other side of the coin is why couldn't Priest find someone more accomplished to fill in for him? I guess they left it up to Ian Hill and those guys probably came up and played a lot of pubs together in the day? Yeah, that's probably it and also explains a lot of the 80's Black Sabbath lineups as well.
As far as Steve Harris replacing Dennis Stratton with Adrian Smith, Harris had his eyes on the prize in the early 80's. The same as sacking Di'Anno. They would have continued being a functional, kick-ass band of the time with those guys but when you listen to Harris' words, he wasn't completely happy with it all until NotB/Piece of Mind. It's understandable because he hasn't deviated from that format at all since then, even given the unfuckable-ness of the S/T album and 'Killers'.