This is an excellent question. It is important to know that not all plastics are created equal. For instance, that 4 mil and 6 mil Construction grade polyethylene sheeting in the construction aisle is the lowest grade of polyethylene sheeting available on the market at any given time. It is made from up to 25% post consumer recycled goods, reground resins, and the absolute least expensive resins that can be bought from any source in any location across the globe. These films are not guaranteed to be pinhole free, and can have quite a few defects that are acceptable in this type of film. The quality control for this type of film is the lowest in the plastic's industry. While it has been an acceptable practice to use quite a bit of this type of plastic sheeting in the past as vapor retarders, that has been changing over the last couple of decades to a very large degree.
Today most experts feel very strongly that engineered films designed specifically for use as vapor retarders should be used as vapor retarders. These products are made from 100% virgin resins, and have excellent quality control. Most are tested to a standard called the ASTM E1745, which is the standard for vapor retarders in contact with soil or granular fill used under concrete slabs. These materials are engineered not to decay in this type of application, whereas most construction grade poly will begin breaking down almost immediately and will have lost most of its structural integrity within 2 years. How many times have you dug up plastic sheeting in your life that is just destroyed from being buried in the ground? I know this is a bit off subject here, but the principles are even more important when dealing with vapor retarders for hardwood flooring, which is considered a critical application for vapor retarders.
It makes some difference what part of the country you are in, what kinds of humidity levels are common in your region, whether or not your home has a good quality vapor retarder under the slab, basement, or in the crawlspace. If you already have an excellent vapor retarder, then the one you use under your hardwood floor is far less critical. If you are putting a hardwood floor over a damp crawlspace, and use a poor quality vapor retarder, you are asking for a lot of problems with the hardwood flooring, and will likely suffer problems with buckling, warping, noisy flooring, lots of expansion and contraction, and often finish problems, just to name a few.
Should you be in an area where Radon is prevalent, or methane, then the use of a barrier with capabilities to retard these gasses becomes important also. Again these films are engineered to have a very tight cell structure capable of keeping these very small molecules from passing right through the membrane. These gasses, and to a very large extent water vapor pass quite easily through the cellular structure of common construction grade polyethylene (aka Visqueen).
This is kind of a long answer to your question, but the plastics in the flooring aisle are hopefully engineered to be better vapor retarders than the ones in the construction aisle. If one wants an even better vapor retarder, then one might want to contact a plastics company specializing in vapor retarders so you can find out which is most appropriate for your application. The vapor retarder is fairly inexpensive insurance for having a great experience with a hardwood floor as compared with what can be a nightmare experience when the wrong product is used.