The problem with the long intake tubes is that you are limited by the volume, compression, and stroke of the pistons. As the pistons lower after the exhaust stroke, they create a vacuum in the cylinder, which draws air in through the carbs (and any leaks).
The ability of the piston to create this vacuum depends on the length of the stroke, how well it is sealed around the rings, and the physical size of the piston. With all else equal, the larger the piston, the larger the vacuum/pressure difference; the longer the stroke, the larger the vacuum; and the lower gap between the piston and cylinder by way of the rings, the larger the vacuum. So for the most power, you want a large diameter piston, a long stroke, and high compression; however, there are limits due to cost, ability to manufacture, the amount of heat through the combustion process available in the fuel, and just the shear size and weight of the engine...but I digress.
So basically, there is a limited amount of air-fuel mixture that can be sucked into the cylinder, and you want as much of that mixture as possible. If you go and make a long intake, you will have to suck up all that "empty" (fuel-less) air in the tube before you get any air-fuel mix. This is OK (but not good or great), and you can compensate by setting your carbs rich so that it will mix with the stagnant air in the intake to create the proper mixture once the intake stroke is done, however, you will end up with fuel condensing on the intake pipe.
The real problems come when you use a single carb for two or more cylinders. Especially with the "out the side" examples shown in this thread. Now one side sucks more fuel-less air than the other, so as someone already said, you will have one side running richer than the other because the ratio of air to fuel will be different.
The compression might (will) be different in each cylinder. Sometimes a little, sometimes a lot different, so there again, even if you have a symmetrical intake using one carb, you will be running one side rich and one side lean relative to each other. If your compression is very close in each cylinder, this will work fine.
To stop fuel from condensing, to keep each side running at the same air-fuel ratio, and to keep the most stable running condition that depends very little on ambient humidity and temperature, you want your intake to be as short as possible, as symmetrical as possible, and you want the same ratio of fuel and air going to each cylinder. That's why stock boots are so short and why there's one carb for each cylinder on stock bikes.
If you want more power and a ram-air effect, attaching U-shaped pipes on the inlet side of the carbs that face forward on the bike is your answer, not doing it on the outlet side.
Source: Engineering, Thermodynamics, Physics