Richard Russell wrote:
Except that they weren't - at some
point during the build the connection was broken and the only
practical way to restore operation was to wire them into the lighting
circuit instead. The electricians assure me it's perfectly OK to do
that, but what are the pros and cons of having a separate breaker for
the smoke detectors?

I believe some interlinked smoke detectors (Kiddie made the one for
which I was reading the manual) require all linked detectors to come off
the same circuit, if you have them both upstairs and downstairs do you
connect them to the upstairs or the downstairs lighting circuit? A
separate breaker solves this problem.