I lived in Maryland in the '80s, between D.C. and Baltimore,
and I agree, it was nice to have both; particularly when
WMAR (NBC then) was carrying an Orioles game and WRC
had the NBC network show. Likewise, WUSA pre-empted
some CBS shows (especially in daytime) that WBAL (CBS
then) carried.
For me, it was best when I attended the University of
Georgia in the '70s. At the time, the local cable system
brought in Atlanta, Greenville/Spartanburg/Asheville, and
Augusta stations. WSB was an NBC affiliate then, and much
more prone to pre-emption than it is today with ABC, so
it was nice to have NBC from Greenville, SC, which rarely
pre-empted. (It also gave us a lot of choices as to when
to watch the network news; ABC came on at 6 from Asheville,
6:30 from Atlanta and Augusta; CBS at 6:30 from Spartanburg
and Augusta, 7 from Atlanta; NBC at 7 from Atlanta and Greenville--
no Augusta NBC station on cable. But that's another story, off-
topic.)