Fire code requires that fire alarms be networked to alert people in all rooms in the event of a fire. Older un-networked units are grandfathers until you begin replacement. Wifi networking is allowed by the code, but for obvious reasons, the inspector has to verify that it works and is set up correctly which requires running wifi.

Personally I would not purchase a home that was configured with nest or other home automation equipment for several reasons.

1) It's a huge security risk. When you buy a home the first thing you do is change the locks because you don't know who the previous owner may have once loaned keys out to. Now you have a bunch of computers in your house giving remote access that may or may not be giving other people access and you can't tell.

2) Most consumer home automation systems have incredibly bad failure states. Those wirelessly networked alarms? When you cheap Chinese wifi router catches fire from a faulty power supply the failure mode is that you die. That's a pretty big oversight. Remember when NEST pushed an untested software update remotely and caused dozens of houses in the Northern Midwest to lose heat in the middle of winter and then be destroyed by broken pipes?

3) Vendor lock in. I don't want the previous owner to lock me into a line of products. Period. I'm sure everyone here remembers when Phillips decided that their smart light sockets would embrace DRM and only accept Phillips brand LED bulbs. They backed down but it is bound to come up again. Not to mention simple incompatibility between competing products and stacks.

4) It's just a damn pain to use most of the time for gain that can be measured almost entirely in novelty factor.