Using live preview means that the camera must be held out in front of the photographer... This is not inherently an especially stable way to hold a camera- especially if the photographer has to hold it that way for a while- because it offers very little additional support for the arms... Pulling the camera up close and using a viewfinder allows the photographer to stabilize the camera because it is closer in to the body. Additionally, live preview becomes more unwieldy the heavier the camera equipment gets, and DSLRs by their very nature are much heavier than the typical P&S... Add a large lens on the front of the camera, and it becomes nearly impossible to use live view handheld.

The other major drawback of live view is that because it requires the mirror to be locked up, it means that the auto focusing mechanism is unavailable. Live view actually uses a different AF technique- one that is quite a bit slower than a DSLRs main autofocus. Obviously this means that in environments where it is necessary to focus quickly and accurately, live view doesn't make for an ideal situation.

Where live view comes in very handy is when taking pictures where it's difficult or impossible to get ones eye up to the viewfinder. A good example of this might be the photographer at a wedding who wants to get shots of the crowded dance floor. Prior to live view he would simply hold the camera above his head, snap off pictures, and pray that some of them had decent framing. With live view it is actually possible to frame up a shot while holding the camera over the head on shots such as these.

Additionally, live view can be a great option where there is no need for the stability needed by pulling the camera in close to the body, such as when it is on a tripod, and/or where there is no requirement for speed inthe autofocusing of shots.