The greatest issue I have with plastic is that it is not as durable as metal. Period. And I am extremely disappointed to say the least, at Nikon for taking the cheap route and making the bayonet mount out of plastic on some lenses. That is something that the cheap lens manufacturers do, not someone with Nikon's reputation for quality and durability. I have used Nikkor lenses for well over 30 years and the one thing that has always separated the Nikkors from lenses made by Canon, Pentax, Olympus and Minolta has been, for a lack of a better term, the Nikon "feel". The others tried but never achieved it. Nikon lenses have always felt like a precision instrument. I used to show people how well they were made by taking my 300mm f/4.5 AIS Nikkor, turning the focusing ring to the minimum focusing distance, placing the lens on a table and watching the weight of the internals turn the focusing ring smoothly and evenly until it stopped at infinity with an audible click. THAT is precision. I don't mean to offend anyone here, but to me Nikon's plastic lenses feel like cheap garbage to me and I wouldn't give you a nickel for an entire truckload of them. Their metal lenses are still much better but the traditional Nikon feel, even in them, is gone.
Sorry Dave, but I have to respectfully disagree with you that a lens is judged solely on its optics. That is simply not true. What good is a optically excellent lens if it is prone to wear or failure because it is made of plastic? Back in the early 80's, Nikon in an attempt to save a little money, changed one of the gears in the MD-2 motor drive from brass to plastic. It was now brass on plastic with predictable results. The plastic gears stripped at the higher frame rates. Subsequently, they changed it back to brass. I can tell you without reservation that if I am still doing photography 20 years from now and Nikon has not screwed their customers over like Canon did when they changed their mount, that my all-metal Nikkors will still be up to the task. How many of the plastic lenses of today will still be in use 20 years from now, or how many of them will be buried in the back of closets, up in attics or buried in a landfill? To me the answer is obvious.
The introduction of automatic focusing lenses has been a Godsend for me, because now I can get manual focus Nikkors, made to the traditional Nikon standard of excellence, used for prices that are a fraction of what they were new because they are not nearly as popular as they used to be. Lenses I could once not afford or justify the cost are now very affordable, and reasonably prevalent. I have gotten several used Nikkors on Fleabay in outstanding condition and although I honestly have every focal length lens I could ever possibly need now, it is still nice to know that if I find a lens I really would like to have for a certain application, I can usually find it in great shape and for a very reasonable price.
It's your prerogative.