Sierra recommends that you don't crimp their bullets with any die-even for AR15's.
I guess I interpret the Sierra Manual different than you. I do not see where they say "don't crimp their bullets with any die".
I see where they admit that the cycling of the action can and does cause the bullet to move forward.
That a handloader has but two options, increase neck tension(does not work) or crimp.
That the Lee Die is a viable option for crimping non-cannelure bullets.
That crimping may not be their first choice for holding the bullet in place.
That crimping occasionally may be necessary.
I have tried the Sierra method of increased neck tension. I have Redding bushing dies for my 7.62x39 Ruger, 30-06 Browning BAR, 300WSM Browning ShortTrac and 300WSM Browning BAR/BOSS.
For example, In my 300WSM I have sized the neck down to .331, a loaded round measures .338. That is .007 neck tension and the cycling of the action still forces the bullet forward 007-.010 (five round tests).
With bushing .335 and a loaded round of .338 .003 neck tension, I have forward movement of .010-.014.
With .003 neck tension and a medium crimp with the LFCD on a 200gr Sierra GameKing(no cannelure) I get .002-.004 forward movement.
I guess this would fall under Sierra's "Occasionally necessary" category.
It is my experience that there is no amount of neck tension that can hold a bullet secure in an auto-loader. That the LFCD can and does hold the bullet more secure, cannelure or not, and accuracy is increased not decreased.