To me, at least, keeping the elbow straight as uke during nikyo cuts down on my mobility in my body as well as opens me up for a technique like wakigatame (which Kevin pointed out).
There are ways of receiving nikyo as uke with a bent elbow that does not force uke to go into a crouching position. One is to "shoot" my legs through to nage's side so that my non-nikyo'ed hand/arm is on the ground along with the side of my body. This prevents the "crouch" that Kevin alludes to and allows uke to retain mobility and abilty to counter nikyo while not opening him/herself up to wakigatame.
As far as having uke turn out in practicing shihonage in the dojo, I think it's up to both nage to effectively gain control of uke's body (eg kuzushi, taking the slack out, extending, etc) as well as uke learning safe ukemi. With someone who doesn't have such ukemi skills (ie someone off the street, perhaps), it's up to nage to make shihonage into an effective technique; the "protecting the attacker" comes in, I believe, after having gained the ability to be effective. Although the option of using henkawaza to change the technique if shihonage fails is a valid option, I think there's a lot to be said about working to have an effective shihonage (or whatever technique) as well.
Last edited by akiy : 10-15-2002 at 11:46 AM.