If your parents werent paying the electricity bill then you'd care more.
For a desktop, MMM will argue who cares, and to an extent he is right. Except with three systems running in a 10' x 12' office, it can get toasty in there if the door is closed.
For DT it really isn't that big of a deal, for system builders it can be as they need to engineer thermal solutions and that costs them money. For enthusiast and DYI builders, heck if you can't figure out how to cool your system you shouldn't be building it.
But in terms of your electric bill, ehhh maybe 10 or 15 bucks a month, equivalent to 2 or 3 trips to McDonalds, who cares....
For the push into the digital home, it matters a lot... example, Television sets (digital) will come with VIIV technology built in shortly, nobody wants to hear the "buzz" of a fan while watching the next American Idol yoddle it away.
For server, it is a huge deal....it actaully costs thousands and thousands to maintain servers, the lower the power the lower the costs. This is where the electric bill really goes up
For mobile, it is crucial for battery life, the mantra there is "how low can you go, how low can you go".
Rambling, I know, but power is really crucial when you account for the big picture....
Try having 1 eMachine in a 14' x 12' room in the middle of Summer...I literally had to LEAVE the room, go into another, and wirelessly Remote Desktop wth my laptop it to use it, I couldn't stand the heat.
Yes, I know of all the corporate concerns and the like, I only argue TDP concerns in Gamer/Power-User Desktop PC's.