One of the IMPORTANT components of smokeless powder is the AROMATIC character. Put your nose to a freshly opened bottle of powder, and smell the powder. Should have a strong chemical small to it. If you sprinkle that same fresh powder out, and allow all of that aromatic component to evaporate, the powder won't have nearly the strength to it that the fresh powder had. Leaving powder in the dispenser is the best way to LOSE potency. It's the actively aromatic components that interact and decay the plastic from your dispenser reservoirs.
So, in addition to hurting the plastic, you will lose potency. The darkened areas that you see on older plastic dispensers is the evaporative component getting into the plastic molecules. You'll get some of the darkening even if you put your powder away every day. However, the dispensers will last a lifetime. I have some green plastic RCBS dispensers (Uniflow) that are 30 years old, and still look like new. They get emptied after each use.
Also, modern smokeless powder is only slightly HYGROSCOPIC. Meaning, ability to absorb moisture. It's the reason we have to constantly re-measure the weights of the powder we dispense. With slight absorption of water vapor, the powders sometimes become CLUMPY. Not good for reliable dispensing.
So, a good general rule: When you are done reloading, put your stuff away, INCLUDING the powder. It only takes a minute to refill, and settle the powder the next time you use it.