Nondirective meditation is a very beautiful and useful, and beneficial form of meditation. It is easy to begin, and it is easy to make into a habit, because there is this lack of you having to do things that are stressful - like concentrating and focusing.
In concentrative styles of meditation they tell you to "concentrate", and you go into it with that as the first thing - the first instruction.
In mindfulness, you "focus" on what is happening in the present moment.
In Nondirective, you "pay attention to" or "give preference to" something - typically a mantra, but it could be something else.
All of these things are really just different ways of saying almost the same thing. Is it really that different from a process perspective?
Whether you "focus on", "give preference to" or "concentrate" on something, your basic instructions are the same.
I think what Nondirective is saying, and why it's been so easy for beginners to pick up - is that from the start, lets not stress too much about the rules. Let's not lazer focus on anything, and lets not "avoid" or "get rid of" anything. Lets just do this thing, and see what happens.
I think that is the spirit in which the other methods, mindfulness, and concentrative meditations were based, and they probably got more strict as time goes on.
Nondirective - from the very start, keeps you chill, and allowing - equanimity is a big part of it. Allowing whatever to happen, without judging it, or wanting anything else to be different.
So, I believe that any meditation instruction could be improved by wording - and some already are being improved in this way. The americanized beginner meditations in mindfulness are already starting to say things like, "Pay attention to your breath", and folks like Jack Kornfield are fond of comparing the process to training a puppy - to illustrate the gentleness that's required.
In a nutshell, I think we can stop arguing so much about which methods are best, and bring the gentleness to our practice that Nondirective allows for. Some folks don't like concentrating on their breath, and some don't like mantras. Some can't get used to the emptiness (at first) that the no-mantra and no-breath meditations require.
Pick what is best for you, and take that gentle spirit from Nondirective, TM, NSR, Vedic meditations, and apply it. I think you'll go a long ways with this tactic.