Moving an arm with a straight elbow is asking for a shoulder injury. Most athletes over develop their muscles and habitually straighten their joints. A straight elbow adds leverage to whatever torque one puts on the shoulder joint-- do this frequently and the shoulder will get lots of small rips and tears.
If you straighten your elbow in motion your shoulder muscles will get bigger to protect all the soft tissue from the regular damage you are doing. Keep it up and you will accumulate lots of scar tissue.
Compression bands stop one from straightening a joint and also help weaken over-developed muscles.
But I have to ask the question; why not just train right the first time?
Most martial artists know not to straighten their joints, but some schools are lax about it. Some schools even teach peoples to straighten their joints. I knew a taijiquan teacher who didn't speak English very well and was constantly telling students to straighten their fingers. The teacher probably meant lengthen.