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

Well, it’s free to try assuming you have a pair of decent interconnects available so just see what you think. 

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.

Your exposure most probably has a decent preamp section and a lackluster power amp section. The accuphase could be an upgrade.

When its power amp section is not in use, i.e., leeching off the power supply for the preamp (assuming they share the power supply), the preamp will also start to sound better.

You can put the streamer, dac and preamp in the same box. Guys like Borresen will tell you that it is in fact a better idea to do so. In many cases, having a seperate everything (streamer, dac, preamp, power amp strung along with unnecessary cables) will sound lousier in the end. The power amp is essentially the only thing that should be separate.

Given the Accuphase  has gain settings you can find a level of gain that matches well with the preamp stage of your integrated. I also believe the Accuphase should prove to be an upgrade.  Much will depend on how the Accuphase mates with your speakers sonically to provide the presentation you desire.  What speakers do you have?

805D3 and Rel S510