What are your thoughts on active resistance from uke during regular keiko?
Resistance, to have any beneficial effect, must be both logical and quantitatively appropriate. Logical resistance arises naturally as a consequence of flawed application of a technique. For instance, if nage fails to take uke's balance then a logical consequence of that failure will be uke being able to resist nage's attempt to finish the throw or immobilization. Applied in this manner resistance becomes another teaching tool offering the student effective feedback on where improvement in application is needed.
For resistance to be valuable as a teaching tool, it must be applied in an appropriate manner for the level of nage's experience and ability. This should be self evident to any instructor and needs no further elaboration.
From what I have seen and experienced over the years most of what passes for resistance on the mat consists of uke grabbing or executing a blow and ceasing all motion. Never mind that uke is open to any number of lethal responses from nage, not moving or being moved becomes an end in itself. As long as nage is limited to the technique being practiced, a sufficiently large or experienced uke will have the advantage of foresight and will usually be able to maintain immovability. This type of behavior is stymies learning and is not permitted on our mat.