How good is to use integrated amp as preamp with the preout feature

I am using it with the same combination, its been great but I want to know if there is something I am missing or messing the things

The preamp in an integrated amp is built to mate with the power amp section, in terms of impedance, etc.  The pre-outs are a convenience.  The preamp section may still be a match for external amps but it may not be. A listening test is really the only way to tell. 

if the Scenario is that I am not biamping with integrated and power amplifier then it should be fine right as the gain issues will not be of any concern and only power amplifier will be driving the spearkers.

The best thing to do is to try. You wont hurt anything. Be sure to have volume down when swapping use of amplifiers.  There are always gain issues. With some preamp/amp combinations there can be too much gain with the result the volume control has too little range of practical use, music getting too loud at low volume settings. 

Does the manual for the Exposure list the gain of the preamp section? What is gain of the Accuphase?

I agree and my accuphase powe amplifier has the gain settings. So should I go for a seperate preamp or this is good enough. I really dont understand why preamp is expensive but the integrated are not even though the preamp section of the Integrated has a very low circuit length as compared to external wires connecting to the power amp.