My knowledge of TV history is virtually nil, but I'm not convinced that what we saw would have been rare in general terms. I mean, use of incompatible footage does save resources, and many a TV show no doubt has borrowed heavily from the archives when pressed for time and money. Did Trek really do anything exceptional in that sense?
"The Menagerie" doesn't impress with its dramatic values. "The Cage" is a story that deserved to be aired, even if in this somewhat truncated form - but the framing story is a senseless collection of unconvincing transitions and implausible actions, and the extremely thin story there only serves to undermine everything "The Cage" stood for.
Which I guess is a bold dramatic twist as such. I doubt, though, that the concept of Pike making a complete 180, embracing illusion and sacrificing reality (not to mention providing the Talosians with the slave race they desired) was really intended and planned for - it just happened when the writers threw together this mess, a clip show among the innumerable clip shows of the modern TV world. Were those less common back then?