The people say they are not needed generally say that because they say its mostly not oil the can has in it. Reading various threads, you'll see people say that the can heats and cools causing condensation. They say that even though there is some oil in the liquid you pour out, it's not enough to amount to anything. I used to run cans on my engines and I would pour out about a half a quarts worth of liquid during the OIC. However, I did not loose half a quart of oil during the same time frame. With that in mind, I stopped using them and said screw it. Right or wrong, that is the way I went.
Even if there is only a small amount, a can will help keep it out of the engine. Its not going to hurt anything to run it unless you forget to dump it.
Will it make a difference long term, who knows.