Because I seem to remember someone saying that battery ones are better? We currently live in a rented house that has smoke alarms wired in to the mains and they are a monumental pain in the a**se! Even before we moved in the one in the kitchen had been deactivated because it obviously went off every time the oven was on. We lit some candles on Christmas Day and the dining room one went off (and wouldn't stop for ages).
We are moving again to a house that does not have them wired in but I get the feeling that they are going to start putting them in to comply with some tenancy law. I'd rather they didn't. Wondered if anyone knew for sure whether they were a legal requirement? (I'm in Scotland by the way)
I would hasten to add that we have a load of battery operated alarms which we put in every part of the house and check regularly.