Moley, I think that we don't concentrate enough on the meaning of the moves - or possible meanings, as moves can be interpreted as blocks, strikes etc. From my viewpoint, I tend to try to learn a new kata because it is required for my next grading. This has the effect of me "forgetting" the one I did for my last grading. It certainly does not help me focus on what it is the kata is trying to convey, in fighting terms.
However, there is only so much instructors can cover in a hour's session. Kata also has to fit in with the other aspects of karate - kihon and kumite - that students want to learn so that they progress.
I am not sure what the answer is to this, however I think that we (students) do have to take responsibility for our own learning, and not have unreal expectations of what instructors can get over to us if we are training 1 or 2 hours a week.
What about asking students to demonstrate one aspect of their current (or previous) kata that could be applied in a real life situation? - That might get us lot thinking a bit more about effectiveness and venom.
Dave