The Dayton situation may also have been a case where the two established VHF stations were willing to give up the stability of being the exclusive sources of NBC and CBS in exchange for keeping a full-time ABC affiliation away from WKEF (or the other new U, WKTR). And ABC was complicit, preferring to put its bigger shows on WLWD or WHIO than to give a full-time affiliation to WKEF or WKTR.
That eventually became too blatant for the FCC, which stepped in and ordered ABC to offer a full affiliation in the market... which then led to a fight between WKEF and WKTR that ended up destroying WKTR in the long run.
It was a pretty brutal business up there on the UHF dial back then.
I've wondered, too, if WLWD and WHIO were trying to carve out a reason for people to view them instead of Cincinnati stations, and time-shifting and mixing up schedules might be a means to that end, to allow a greater diversity of programming in any one daypart or part thereof, especially in prime time. Only the northern part of the Dayton market could get Dayton and nothing else easily OTA (but then there was WIMA/WLIO), Dayton proper and everything south of it could get Cincinnati with no problem, also, Columbus could probably have been received with a better antenna in Dayton, and even more easily in Springfield and the eastern part of the Dayton market. I don't know, I'm just guessing.
It also works the other way around --- if viewers in Cincinnati could receive Dayton without working up too much of a sweat (the Cincinnati area is more hilly than the Dayton area, where things start to flatten out into a more typically Midwestern lay of the land), ditto viewers in the western part of the Columbus market, then those viewers, too, have more diversity of programming in any one daypart, if they get creative enough to turn to Dayton stations, or just get curious and say "hey, what's on 2 and 7 tonight, anything different?".
It sounds convoluted, but just pick up any TV Guide at random, whether Southern Ohio or any other edition, and note that schedules often vary significantly from one market to another, time-shifting, pre-empting, and so on. That was one of the appeals of cable providers carrying duplicate network affiliates --- schedules from market to market weren't stamped out of a cookie cutter as they are today. In our time, pre-empting is rare, almost all markets have full-network service (using subchannels in smaller markets), non-network programming is basically all the same, only different channels (finding what channel
Wheel of Fortune and
Jeopardy! are on when you're traveling is a thing, as well as that "7 pm vs 7:30 pm ET" situation), and the only thing that varies all that much from market to market is local news. Gone are the days of "cool" independents with syndicated offerings, reruns, and huge movie libraries, WXIX, WUAB, WTTG, and WTTV come immediately to mind. That role is now filled by subchannel diginets and various cable networks.
TLDR, the Dayton market had a lot of overlap, and WLWD and WHIO may have needed to carve out some reason for viewers to watch them instead of Cincinnati or Columbus, aside from local news.