Dream! Why? Because they are the foam padding on the floor which save us from the harsh reality, as we try to maneuver the acrobatics that will lead us through the real world, there is nothing wrong in day dreaming, if you know that they will help you only if you work to make them real and of course the world acts in favour of your toils.
Dreams and day-dreams often dull the blow of disappointment of failure of one project, they instantly pop up the hope balloon that says there are other things to try for, it is but one project.
Apart from that, you can be pretty sure that when man aimed for moon that too was called daydreams of a lunatic!
These days scientists are saying daydreaming and dreaming are signs of intellect and creativity. So you can daydream but do keep your feet on ground, or else they can be absolutely destructive, in place of acting as a cushion they will become quagmire, suck you alive.