It probably is cheaper to manufacture them but, the pertinent question is, does it make the reel a lower quality offering? Many years ago, people complained when (automotive) suspension pieces started being made of aluminum. For many, 'aluminum soda' cans were considered too weak a material to make car parts from. Time has proven that properly engineered, aluminum suspension parts are often a better solution than steel. Aluminum, like plastic, comes in many formulations with different characteristics. Just because there are plastic parts in a reel, I'm not going to write it off. Of course, all are welcome to their own opinions but that is mine.
I might need to eat those words if my new K doesn't prove durable.