First of all, details as to detector placement are usually found in your local building code; detectors also come with some pretty comprehensive instructions. There is certainly no harm in having a detector that might be in addition to the 'code minimun.'
Even if at the same level, one can argue that a detector placed, say, over the living room would not be able to qualify as the one required 'in the hall outside sleeping areas.'
Secondly, the 'third wire' interconnecting smoke detectors is a fairly recent addition to the codes. Originally, detectors could be battery powered only, or line voltage only- not the 'battery back-up' now required. This house may very well have these earlier detectors.
There are also a detector or two out there that 'communicate' in ways that do not require that third wire.
I don't believe smoke detectors are required to be on dedicated circuits; if you want a light up there why not just add one? It can't be any harder to replace a detector than to hang a lamp.
