Its interesting also that you bring up trouble with students who jump prematurely into technique... I recently had a conversation of a similar nature and I think there is some solid argument to advocate a stronger awareness training program prior to getting into technique...
As I said, internal strength skills require learning how to do them and then they take some development and conditioning. If you're trying to change the mode of movement over to more than just some coarse jin, but also into the ki, hara, etc., full-blown usage of internal strength, you have to convince the body to re-pattern itself. Any practice which is counter-productive to that re-patterning is just time wasted. So a person doing suburi with normal strength (even worse with a heavy suburito), weight lifting using normal strength mode, application of complex (or even simple, I guess) techniques (normal strength and prior training in those techniques can't help but kick in), and so on, is wasting time. Is swinging a moderately heavy suburito "tanren"? Not really.
BTW, if there's any one exercise in Aikido that is excellent for repetitive i.s./tanren development, it's suburi, IMO. Great exercise for ki, kokyu, hara, breathing. All the worries about angles, wavering tips, etc., are good, but my suggestion would be to forget those parts and use the suburi purely for tanren of kokyu/hara, etc.