It has limited usefulness for framing and checking focus. The main limit is
that the viewing screen can never be large enough to be accurate enough to
check critical details and it is worse in daylight. If you need to check
these things the camera needs to be tethered to a computer in a studio.
It is of limited use to a knowledgeable photographer but has its place, not
indispensable.
However, as implemented by Nikon, with the shutter open and continuous
output off the sensor you now have a dSLR with HDTV recording capability.
In a camera with the specs of the D90 that is a real enticement.
If I still had a retirement plan . . .