Being that none of us have AppleTV yet it's hard to compare 1:1, my reasons are below.
My iMac is connected to my Samsung HDTV by mini-DVI to VGA adapter and it works very well. There are some risks in plugging in a computer into the HDMI port as, though DVI and HDMI are oin compatible, the signals those pins carry is a bit different. So most HDTV manufacturers do not support that use. That's not to say it won't work, but it might not be worth the effort in getting the resolution and overscan settings right.
I've got an Apple TV on order, even though the iMac is "right there" for three reasons.
1) Lets someone in the family use the iMac while another person is watching AppleTV. Not so much of an issue if you're the only user.
2) No need to switch around the primary display settings to get iTunes/FR to work on the TV.
3) Audio that makes sense. With the iMac set up as it is I usually use the iMac's speakers when watching TV as it makes for an easy switch in/out of the different uses of the two devices. If the audio is hooked up the TV and someone changes the channel, you have to unplug the cable to get audio from the iMac's speakers.
All of those are usability improvements that'll let my kids view their media more easily than asking for help about how do I get what's on the computer on the big TV.
B