Why would a sprinkler system fail in a high rise? A sprinkler head has a glass plug with liquid in it that causes the glass plug to break once it reaches around 165°F. Thus, releasing the water. In high rises, there are also mandated reserve water tanks at the top of the building to ensure constant flow pressure.
They even mandate it in smaller apartment buildings now in Detroit. I was looking at a one-bedroom apartment in La Vogue in Palmer Park and saw 8 sprinkler heads inside that one-bedroom apartment. I asked why they would need it on a building this size and they answered that it's now required in the city by law.
As for Granny's homeowner insurance rates, why doesn't she get a sprinkler system if a fire policy is why its skyrocketed to $4,000+ a year [[it's only 50 cents to $2.50 a sq. ft to install).
http://www.doityourself.com/stry/firesprinklersystem