From the numbers presented for Thin Clients they may seem more cost effective and green, but those numbers or for small computing tasks that do not task the servers. Unless everyone in an office enviroment is a power user at the same time, the machines will not be not be taxed as much as just 2 users streaming HD content. Even with home automation the system can be taxed, with the processing being done by 1 location, the server can be overworked as it trys to supply data to multiple UIs and devices for control and monitoring.
Once the system is overworked, then the server will require more power negating the greeness of the thin clients. While I understand the benefits of Thin Clients, I also know the down side to how they work. Using a client that does the processing can be a better way to go in most cases. This could be a reason why they are not used as much as smaller PCs.
Not saying that Thin Clients should not be considered, just saying that only by looking at all the facts can a system be created with the proper equipment for the job.