the point of the relay (as thefdog mentioned) is to ensure that power [to your pc, monitor, cig lighter, etc] is cut when car is off. you can still use a switch with a relay to only apply power when you want it. the relay simply ensures that you'll never have a dead battery because you forgot to switch power off to all your devices.
your plain old on/off switch will work fine (as long as it can handle the current that all those devices will pull from it), but you'll have to remember to switch it off when you leave the car. otherwise it may drain your battery. with a relay you'll never have to worry about that.
from this thread and several others you started (they are basically all about the same topic) you seem very reluctant to add a relay to your setup and I'm not really sure why. it will only make your life easier. they only cost $5 and it's only 2 extra wires
