I had my season ticket in the Platt Lane from 1980/1 up until it went all seater in 1985 and was very pissed off with the fact. I understand fully that we're talking about the days when hooliganism was still rife, and the previous arrangements were difficult for the Police to administer: the away fans used to have a standing area in the Kippax at the Platt Lane Corner and seats in the North Stand/Main Stand corner, which obviously wasn't ideal. But to remove City fans from a whole stand which had at that time, IIRC, a capacity of 8,000 was a joke. The club should have divided the end if the away fans had to have seats in there (and it would have been possible), but obviously couldn't be bothered.
A couple of things happened as a result of that decision. Firstly, I think we lost some fans. Certainly, if the reaction around me was anything to go by, many ST holders towards the end of the previous season were swearing blind that they'd stop going rather than be forced to move from their regular spot. And secondly, it affected the atmosphere at the ground badly. Only United, Liverpool and Everton tended to bring enough fans to get close to filling that stand. Even when away teams had a couple of thousand in there, it looked pretty empty. And most teams in those days (post-Heysel and Bradford) brought puny numbers of fans, making it like playing in a three-sided ground. Look at footage from the famous 3-3 draw against Bournemouth for an example of a ground pretty full on three sides, with virtually no one behind that goal.
As for derby tickets - well, as noted, United prized allocating tickets to their own fans first and foremost, while Swales and his mates were happy to cram as many United fans in as possible to take the revenue from a bigger gate. Simple.