I think part of the reason is to prevent:
[Homer Simpson doing his Mr. Burns imitation]
“Hello, I’m George W. Bush and I’m a big smelly monkey. Please vote for me so I can eat your children and drop a nookular bomb on San Francisco [sub]paidforbyjohnkerry[/sub]”
“I approve this message and so will everyone who doesn’t hate America. You don’t…Hate America…do you?”
I am Rudy Giuliani… DO WHAT I SAY… I am Rudy Giuliani… 
“I approve of this message so much,… I bought the country”
My name is George dub-ya Bush and I approve this message.
<click>
My name is George double-u Bush and ice approve this message.
<click>
My name is George double-u Bush and I approve this massage.
Wait a minute, guys…
<click>
My name is…now wait…
<click>
I am President George double-u Bush and I approve. Cut!
Announcer: Reelect George W. Bush. He means well.
“I’m a George Dubya and if I don’t prove of this here message I’ll eat a bug”
It’s a new law this election that’s supposed to raise the tone of the mudslinging. The idea being that a politician won’t accuse his opponent of being a child-raping pig-kissing Scientologist if they have to attach their name and face at the end.
“I’m Dragwyr, and I approve this posting.”
“I’m Dragwyr, and I approve this purchase of Preparation H.”
“I’m Dragwyr, and I approve this order of one Big Mac and an order of Fries.”
Um, I think the OP is saying that Kerry is the first person to morph the “I approve this message” portion to an actual substantive part of the commercial, as in “I approve this message <plus a bunch of other stuff I approve of>”.
Hello…I’m…WilliamShatner…and…I…approvethismessage.
KHAN!!!
“I approve this message as a healthy part of a nutritious breakfast!”
I’m George W. Bush, and Almighty Gawd approves this message.
I’m George Bush, and I approved this message. I just didn’t understand it.
I’m John Kerry and my wife approves this message in five languages, but not all that fluently.
“I’m George Bush, and Dick Cheney approves this message.”
I’m George W. Bush and I approve of this message. If you don’t, then you support terrorists.
I’m John Kerry and I approve of this message with extra ketchup. [sub]Buy Heinz[/sub]
. . . and I approved this message because my media consultants told me you’re stupid enough to believe it.
“I’m George W. Bush, John Kerry is a stupid dickface, and I approve this message.”
“I’m John Kerry, that’s right, John Kerry - NOT George W. Bush - and I approve this message.”
“I’m George W. Bush, and I approve this message because ketchup is a vegetable.”
“I’m Ralph Nader, and I don’t approve of anything.”
I’m Liberal, and I approve this message in the context in which it was originally intended, before the two-party system co-opted it for their own nefarious purposes.