I multi-task as much as the next 21st-century person - probably moreso, considering I earn my living working on the Web, and my company's corporate culture leans heavily on instant-message communication (who can wait for e-mail? :-) ) Many of us have multiple IM conversations going on at once during the day, even when answering e-mail or talking on the phone, and it's all part of the adrenaline rush of a day at the office.
But one's personal relations can and should use a different standard. Call me old-fashioned, but when talking to a loved one, I think one's full attention should be on the conversation at hand, not checking to see if someone more important is trying to call. We've got voicemail for people to leave a message if I'm already on the phone.
I can see how call waiting can be necessary if you've got kids of a certain age, or some other pressing personal reasons why there are some calls you truly can't miss. I, however do not. One call at a time is just fine. Which is why I asked Verizon not to turn on the call-waiting service we're entitled to as part of our telecom package.