mechanically speaking, there is most likely more compression between the upper and lower arm at the parallel position in a skullcrusher than locking out a 100kg bench press.
The long lever arm will probably create a compression that's 10 times that of the weight you're lifting. In my biomechanics class we did a quit calculation on compression in the elbow during a curl. With the numbers we used (which were realistic) holding a 10kg DB at 90 degrees would create a compression of 900 newton at the elbow, which the professor said was equivalent to putting 90kg (just using 10 instead of 9.81 as the g, don't shoot me) pressure directly on the elbow. This is probably what you'd get from locking out a 90kg db in a DB bench press or OHP. People don't seem to understand that muscle forces creates A LOT of compression, translation, etc, in our joints all around the body. So the point in a movement where the highest compression occurs is most likely where the highest muscle activity occurs (assuming the direction of the muscle force would cause compression. It could also cause translation, for example the hamstrings in a squat will pull the lower leg backwards, creating an anterioposterior shear force, not compression)
That's the argument I usually use against the "don't lock out due to injury risk" crowd.
on the other hand:
When you lock out, you let your muscles relax a bit, which could perhaps create an instability and maybe overextension, which is not good.
However, I don't think there's a big injury risk as long as you don't "snap" your joints when you lock out.
The argument I've most often heard is that by locking out you put all the weight on your joint and not your muscles, and that this causes injury.. this is simply false, there is a lot more weight on your joints when your joint is in a position where your muscles have to work lot with a long lever arm. The only possibility I can see is that your joint could be more unstable and therefore more suseptable to injury.
Another argument is the TUT argument. Keeping the contraction going will inhibit oxygen from coming into the muscles, resulting in more lactate production, which has been associated with increase anabolic hormone responses. So constant tension could be a good idea for hypertrophy.
