That's kind of my point. Things all have different value to different people, based not only on their interest in the thing in question, but also on their circumstances. In light of this, I think it's very arbitrary and largely futile to try to judge who cares enough.
My instructor used to say, "you cannot hide who your are on the mat." There are a number of indicators to help us assess this subjective decision. We have attendance records, belt ranks, leadership positions in the dojo to name a few. We also have implicitly given some measure of control to our instructors to make this decision - we have an obligation to keep them informed of our personal decisions and such so they may make an informed decision. So we are empowered to make a fair guess at who is training to a level of expectation set forth by their personal circumstances. Of course, we then need to consider the answer and adjust our training accordingly...
I think the problem is not deciding who cares enough. I think the problem is that we often think
we care enough and that perception is inconsistent for our expectation of skill. I think the Western model of buying access to excellence causes confusion. Here we have people who pay to touch the robes of our shihan and other high ranking aikido people. They take our money and train with us. They stay at our house. And they politely refrain from telling us our aikido is poor because we train 2-3 times a week for 1-2 hours, usually. Well, except on holidays, too. You want to train with an instructor who gives tough love? See how many people are on the mat... Sometimes we need to look at alternative solutions. Can't be on the mat? Does your dojo offer solo exercises that you can do at home? Do you do them?
Then there's performance on the mat. Yes, there are some lucky people out there who are just good. Bo Jackson once remarked that he actually did very little to maintain his body prior to college. But most of us have to work at it. Most of us make mistakes. And most of us are not nearly as good as we imagine in our heads (where surprisingly few damsel butt-grabbing, ex-convicts on drugs in a bar escape our imagined, cool, even-handed control until said assailant realizes the error of his ways and enrolls in a self-help clinic).
Finally, sometimes we step away from our investment because it help insulate us from being hurt by criticism. Sometimes, I think students are intimidated by a fear of criticism. The defense mechanism is simply to "not to care enough," to be hurt by the criticism. I think as instructors, we need to always be considerate of what we say and how we say it. I think aikido has experienced a period of instruction that was not considerate, with individuals who maybe were gifted martial artists but poor instructors. We forget there is no "teaching 101" for instructors. I think aligning with instructors who are considerate is important to introducing better means of constructively adjusting our training.