There are deeper levels of using IP/aiki that are never going to be attained without crossing hands with those with IP /aiki who are resisting you with IP/aiki.
I hear this argument all the time and find myself wondering... if a person will "never" attain these skills without crossing hands with someone who already has "the stuff," where did "the stuff" come from in the first place? Chicken? Egg? Tengu?
As far as "resisting" goes, Am I correct to assume that you mean "not letting you move as you wish?"