that what makes dSLR a dSLR.
1. That fast response (no lag time) can only be achieved if the sensor is in capture mode and the shutter does the on/off thing by swinging the shutter.
2. Serious close up, manual focus, checking depth of field, require optical viewfinder, there is no high-res preview technology that can compete with the accuracy of optical viewfinder. I tried the Olympus live-preview technology and for the life of me I can't take a single proper macro shot, can't do an accurate manual focus and...
3. Live LCD preview creates distance between you and the camera, which makes your camera handholding less steady, which in return creates shakes/wobbles while you're taking a picture.
I'm sure if the display technology have caught up with the accuracy of an optical view finder, there will be dSLR with live preview AND electronic view finder
Oh, that's another thing, there is not a single electronic view finder out there that's good enough to do any manual focusing/depth of field control work.