Why is it absurd to consider tablets for phone calls?

Okay, I get the point that holding a 7 to 10 inch tablet up to your face to make a call would be stupid. But in this world of bluetooth headsets and front-facing cameras, why couldn't you use your tablet to make calls and possibly fill in for your cell phone in a pinch?

If you recall, the original Galaxy Tab, a 7 inch android tablet, originally had phone functionality built in. By the time it made it to the US however the phone features were stripped out. Why? I mean, if you have to buy a separate cell plan for the thing anyway, why is phone functionality not included?

I'm not ready to toss out my handset, but using my tablet for bluetooth or speakerphone calls would be a welcome option.