These new ones seem to be adhering a little closer tot he original Ralph McQuarrie drawings, which were hella cool! I always thought when the model makers really got down to actual building of the X-Wing, and asked themselves that same question, the response was "Well, it looked good on paper, but from a practical perspective, each engine needs to have its own fan housing", or words to that effect. So, they made subtle changes, which I thought looked great too.
This time around, going back to the older design is being portrayed as an updated version (T-70 vs. T-65). I still love the design, so we will see how they portray the workings of the engines on-screen, if they even really care to. Might be one of those "it works, because the script says so" kinds of things.