I guess the first thing I'd try to be mindful of is that the concept of mindfulness, as well as its practice, are but a small part of an all-encompassing world-view and life philosophy of Zen, and when separated from (devoid of) the totality of zen it's of little more use then the Resurrection is without the Fall, Original Sin, the Promise, and the Passion.
But in short: it's knowing that nothing anyone said or did - or didn't say, or didn't do - in the past can ever be changed, and all your thinking, contemplating, obsessing, and all that about anything in the past is a useless exercise and only serves to make your present even worse.
That all your planning, scheming, star-trip dreaming about the future tends not to make much of a wave either. Sooner or later the future is going to happen without you even in it. That's how much of a key player you are to the future. So all that shaping, molding, planning, making agendas, working out conversations in your head, all that 'well someday...' is just as much a waste of time as dwelling in the past.
Meditation is little more than sitting down and telling yourself to shut up for a while. No, really, sit down and STFU. Stop that constant internal dialog you have going on. It's not helping you. In fact, no doubt - as it is for most people - its a hindrance to the most basic enjoyment of life. It's that dialog of 'wanting,' 'needing,' and the worst - 'DESIRING' (or as the Bible called it 'covet,' and desire, or covet if you will is like the worst dope habit ever!) - that messes up your life because as soon as you sit down and stop all those voices, ideas, attitudes, notions - all the ->-bleeped-<- people have tried to shovel in there - you find out you don't need any of it. None. Not a thing.
Meditation can aid you in getting to a place where you come to understand those things about the past and the future (and to stop living in your mind in both the past and future) and to see, witness and be a part of the greater understanding that everything you need in life is right here, right now. That this moment is everything for now and forever after. That all of this pretty much is as it is, and we don't get to do much about it. Wanting things to be different - wanting the universe to change - well the universe is more powerful than you are and that resistance eats you alive, because its vast and powerful and you tend not to mean ->-bleeped-<- to the universe, so thats bad on you.
The only way to 'heal' the past (or at least mitigate it) is by doing the right thing in the present moment. Likewise, the only thing that will even possibly shape the future is what you're doing right now, in the present moment.
The only way to live in this exact moment (and Zen and the Tao both agree on this, it's the central unifying concept) is to accept what is in front of you without wishing it to be any different than it is.* Because wanting change only sets up resistance. Peace is found not in conquering, overcoming, or winning - but, rather, in simple continuation.
To this degree (and it's why I think talking about it out of context is like having a transmission to an automobile and not anything else, it's kind of abstract, heavy and not likely to go anywhere on its own) I think that though meditation aids us in focusing on the present (if only because in the beginning it takes all your mental powers and then some to STOP that internal dialog and find out how to be here now) its not a be all and end all. And in that sense, if one is asking if they are 'useful' then perhaps the correct question is not being asked in the first place.
Use and utility, and particularity achievement, are very Western concepts, and as such are pretty much the counter to Zen and Taoist thought. Achievement is just about the most counter Zen concept in the world.
The zen master reply would be, I assume: you use it to get rid of thought about always trying to use things so you can achieve something.. What you should 'obtain' from Zen practice is not wanting to 'attain' anything.
It is very much about being in the minute, being here now, and all that. So if you're walking away from such things with 10 point action adjendas about 'what we have to do' then something is not right, because its not about doing at all. Its really much more about not doing. It's about acceptance, supplication, and the stillness that comes with those understandings, primarily that of: how can I best work with the universe and not against it, how can I be the most useful, what can I do for others? You search for the right thing to do, or be as you feel it, as opposed to as you desire it. And if you do it right, in those things peace, love, harmony/balance, and understanding are found. And indeed, they are not found anywhere else.
* - That's the entire deal, the end teaching, in one sentence.
