Simply put, I don't think they are designed or intended to be compared to each other. They are intended for two very different types of photographers. I would argue that the 5D3 is at least two, if not three, classes ahead of the T4i. Rebel series, 60D, 7D, 5D3, 1D series.. Although, the differences between the 60D and rebel seem to be shrinking..
(I won't comment on the subjective differences in image quality. Huge can of worms.)
First, the sensor is entirely different, so the sensor debate ultimately boils down to the age-old full-frame vs crop, which has been hashed out endlessly in every online forum on the Internet. The main thing is that full frame sensors simply cost more than crop sensors.
Then there's the build - materials, quality, weather resistance, etc. Magnesium alloys cost a lot more than plastic, and it costs more to shape magnesium alloy than injection-molded plastic.. More has been spent in design and testing (although maybe not enough?), and they're making far fewer 5D's than rebels.
There are countless other upgrades from the rebel line to the 5D line, but those are the main two in my mind. The sensor, build quality, materials, and manufacturing scale, mixed with complex economics, all combine to get the prices we see.
Bottom line, yes - it's well worth the price difference for those who need it. Should everyone go buy a 5D3 instead of a T4i? No way. If all you need is a rebel, get a rebel. If you need better construction and fast performance, get a 7D. If you need a full frame sensor, get a 5D. If you need the absolute best Canon offers, get a 1D or 1Ds series.
Of course, if you have the money to burn, by all means - get a 5D3 or better, whether you really "need" it or not. If you can't justify the cost, then there's nothing wrong with going with the rebel. It's still a superb camera, and it will give you great images, but it simply cannot do everything that some photographers need, which is why the 5D3 was made and priced as it is.
