Basically your arms, from the shoulder to the elbow, transmit the power of your hip/torso rotation and your wrists and forearms control your hand speed and bat's swing level through the plane of the plate. Cheat your elbows and you cheat your swing. Its OK for making contact against an overpowering pitcher or when you need to make contact when you're behind in the count, but you don't drive the ball as well. A good alternative is to just move your hands forward in the same stance a few inches.

that is not true
i never put my elbow up, and i get solid hits almost all the time

Not true at all most everyone has different batting styles.

Well, it's different for every batter. For those who prefer to raise your elbow [me], it may help you get better bat rotation and it also helps me swing a level swing.
But if you don't raise your elbow, you can still get great hits. It just depends on your batting style, and how you can work with it.

no, not true. I bat with my elbow basically jabbed into my ribs at about a 45 degree angle and the bat starts out flat. I've never been able to hit for power (I hit a lot of singles and some doubles), so this helps me keep the bat level and get it through the zone really quickly. Gives me a lot of flexibility as far as directing the ball to all fields.

Well, its pretty true for anyone using a regular batting stance... lowering your elbow will cause you to take a huge uppercut unless you have a very short swing, this means you will whiff often and also pop up most times u hit the ball, unless you are really strong. I wouldnt recommend hitting with your elbow down.

Keeping you're elbow up helps with bat speed and getting the bat extended through the zone. Keeping it in you can still hit. As long as all of your mechanics are good you would be fine. Keeping your elbow up is part of having proper mechanics. the better you are in more mechanics the better hitter you will become

you can definately hit it still, but "proper technique" (whatever that means lol) says to keep your back elbow near a 90 degree angle to keep from "hacking" at the ball.

another tip Jim Leyland (Tigers mgr) taught me was to align my 2nd row of knuckles to each other w/my thumbs pointing away from me, keeps from having a slight arc mid-swing.