Nitrous plumbing A1A...??..

No need for a fuel purge. Liquid fuel flows easily since it's not trying to evaporate all the time.
Nitrous oxide is kept at high pressure just keep it from evaporating. In the bottle, the liquid is at the bottom and gas obviously rises to the top. That is why there is a siphon tube in the bottom, so you get the liquid delivered up to your solenoid. But as nitrous sits in the line, even for just an hour, it starts evaporating. When it evaporates you still have pressure in the line as a gas, but no liquid. Now when you activate your system you have ambient temperature gas being injected instead of liquid that instantly evaporates and super-cools the intake charge. I am sure some escapes the system too, but mainly you lose the super-cooling effect that normally happens, so your now your air/fuel/nitrous charge takes up more space inside the intake manifold and slows down the airspeed. Bog, hesitation, and stumbling are the result. Then the liquid finally reaches the solenoid and passes through the jet, hits the ambient temperature intake charge, instantly evaporates which starts super-cooling the charge, and it finally reaches it tune potential and starts pulling hard.
Same thing happens if the bottle pressure gets too low, you lose nitrous delivery effectiveness and it falls on it's face.
So, now that everyone stopped reading this excessively long sequence of posts, I finally get to the point; nitrous purge is specifically to ensure liquid nitrous is ready at the solenoid and not at too high a pressure which will lean out the tune.
I don't use a purge system at all because I don't care to show it off. I actually just do a quick full throttle dry hop while driving to purge my basic plate system. Pittsburghracer will undoubtedly run a purge system on his because the tune will be so critical. He may even need a second tank to maintain high enough pressure to prevent his system going very rich during a run, because of the drop in bottle pressure when that much nitrous comes out of the bottle quickly. Simple bottle heaters don't keep up during a run, they just bring it back up to pressure after sitting a while heating gently.