There are some direct and indirect consequences of this habit. First observation I made is that, although not necessarily always true, people who drop anime don't own it on dvd or bd. If I am going to buy anime, I am more likely to watch it completely through. So if you poke around profiles on myanimelist.net, you can see who the 'real' anime fans are. In addition, people who chronically drop anime are more likely to miss out on a great series that just happened to start slow.
Now I am not saying that you should not drop any anime at all. As Brent of Otaku no Video put it, there is an opportunity cost when you start an anime. If the anime is so terrible, it would be smart to drop it and watch something better instead. For the record, I have not dropped an anime series so far. I usually try to stay away series I won't enjoy.