The 767-200ER in not out of production, Continental has them order with Boeing.
Besides the range difference, an airline may choose a plane for fleet swapping during peak and slow times. For example, an airline may use 767-300 (or-400) during peak times and a smaller 767-200 during slow times. This allows the airline to keep crews the same where if they swapped to a different aircraft, that would require new crew basing.
Someone mentioned earlier Continental does this now with the 737NG flights, use different airplanes for the same flight depending on the day of the week. The 757-300 was choosen by airlines who fly 757-200. I am assuming for them to add extra seats, but stay with commonality.