My research-oriented U also has a teaching-only stream like the one at Janewales's place, but positions open up pretty rarely. The (very very few) teaching-only faculty in my department only teach undergrad courses, and rarely teach the upper-level ones.
Janewales, aren't you in Canada too? Maybe teaching stream is more common here.
Yes, I am. At my place, the instructor stream seems to have gone through various phases. When I first started (a long time ago), many departments that had significant service teaching (think English composition, introductory language, introductory lab science, math for majors in other things) had a few instructors.
The stream began to shrink as the university became more overtly research-oriented; the existing instructors kept their jobs, but no new instructors were hired-- and some of the instructors published their way out of that stream, and onto the tenure track (the framework agreement allowed for that possibility; I had two colleagues in my own department who did it). It's perhaps worth mentioning that many of the instructors were women, often spouses of male faculty, who had been hired to these positions in the 60s and 70s.
Eventually, the teaching load for professorial faculty had to be reduced because of the increasing pressure of research, and so someone had to pick up the extra teaching. As at other places, sessionals filled the gap. At first, they were on rotating contracts; that is, they had to leave after a fixed number of years. Then they joined the faculty bargaining unit, and gained some rights of reappointment, though the positions remained soft money.
And now, as of last year I think, the tenurable instructor stream seems to be on the verge of revival. My sense is that it's a way of dealing with the growing dependency on sessional instructors; tenure-stream instructors are more expensive than piecemeal sessional instructors, but still less expensive than the tenure-stream professoriate. I've seen only a few appointments thus far, and they've been conversions of existing long-term sessionals, so I don't know yet how significant this move will be.
I wonder if Canada is doing more of this because all of our universities are public, and it's important to be able to tell the public that teaching is being done by people with the proper support and resources?