The only thing I can figure is that the early models, which did not have the plastic shroud around the fan, could have been intended to draw air in from either side of the fan (the side door slots extend all the way along the bottom of the side door) and then out through the fan. This air coming in at each side of the fan could have been drawn around and over components in the bottom of the case and also drawn up to the exhaust fan in the power supply. This is only a guess and, since I am making it, I also will guess that it was not a very efficient cooling design.
No, efficient is not a word I'd apply.
With addition of the plastic shroud around the fan and the fan set to draw air into the case, the fan then draws air from all along the slots into the case. This is a much better air movement system and it seems to me the early system without the plastic shroud and with the fan in backward was just one of those early Apple designs they didn't give much thought to, didn't work well, and was soon changed. I have seen Apple put out other designs that appeared to have been hurriedly put on the market and then changed soon after they went into use.
I can see the reason for the baffle - that's pretty obvious. From reading the manual however Apple appeared to do the opposite of what you are saying with the fan. The manual states that early fans are essentially mounted to suck air into the case, but later ones (and retro fits) should be fitted to blow air out. I think this wreaks of early attempts to cool PCs when CPUs first started getting hot (Apple got a head start on this as their CPUs got hotter earlier) which was basically a 'It's got - get rid of it!' mentality that didn't pay attention to airflow cells or systems.
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