whelder is right, if air goes in it goes out. in that case the air would just exit through the rear hole the rear fan would normally mount to (even if that fans not there, its still a way for air to leave), plus whatever the power supply fan exhausts.
Yes, the air will still leave, but the question is where. around the seams of the side panel, holes in the front bezel, etc. Also, if you aren't balancing push to pull, then you have to factor in the air pressure ratings on your fans. As the pressure in the case increases, that same fan will push (or pull) less volume. As an example, see how this article
compares the use of a single fan and a push-pull, two fan set up. The two fans in this case aren't moving any more air, just balancing the pressures, because one is pull out what the other is pushing in. Now, admittedly, it may be a small difference (1degree in most cases), but it is a difference.
a rear fan will slightly improve air flow through the case, but even with no rear fan the airpath will be exactly the same, just a bit less airflow.
Again, I'll disagree. Without the exhaust fan, the pressurized air is going to find whatever way out that it can. That might be cooler air escaping the front of the case, or around seams in the case, etc. However, a rear exhaust fan is going to directly pull out the hot air coming off of the CPU cooler.
Maybe, I'll remove my rear exhaust on one of my systems and check the CPU temp difference, as well as ambient case temp ...?
Id be interested in your result if you do this. Ive played with fans a lot. my best CPU temps was 2 rear fans blowing in, side, top and front out. but my mobo chipset and HD temps were higher. trade offs..
my point is air, like water or electricity, will take the path of least resistance. since theres a big hole there already, most
of the air will exit that hole naturally. a little will exit other ways but its more or less insignificant.
also, to Shinekaze, I have to disagree, I prefer positive pressure as you can control the air going in (like filter it). with negative pressure air (and dust) is drawn in through drive bezels, USB/audio connectors, around the I/O shield etc. dust builds up and it looks nasty, as well as possibly causing problems. with positive pressure you dont get any of that. as for dust settling. just prevent it from getting in there in the 1st place, filter the intakes.