I just recently noticed the YouTube settings option for "Allow advertisements to be displayed alongside my videos" and it looks like default is "enabled". This is pretty sneaky on Google's part, because they don't tell me ads are on my videos even though they are and it wasn't monetizing at all since I started posting videos.
Now, say I did turn monetization on. Does this "increase" the frequency of ads that are shown on my videos (not sure if ads are shown every time even with the ads function on)? I've already subjected my poor viewers to ads without my knowledge, so if it doesn't make any difference maybe I will consider turning monetization on.