Many a show has been ruined due to an overloaded attempt to appease an entire fanbase - my opinion of course. Perhaps I'm bitter due to the overwhelming number of shows I have ceased watching because the story felt like it was falling off of the rails.
But we're going to start with the good news (always good news over bad news, right?)
I'm looking this from the viewpoint of a writer - fanservicing allows writers to get a general survey of not only the amount of people who watch and engage with the story, but to also hear potential plot ideas that viewers could potentially see occur at some point in the series. Theories about canon life and possibly forgotten plot holes that writers might find an intention for. Actively engaged viewers can often catch, and point out, things that writers tend to forget about. (A writer can only hold so many storylines in their head at one point after all.)
There are some bad things to it - unfortunately.
One - the intent in fanservicing is to please loyal fans. We all know that you can't please everyone though. Two - in the attempt to please fans, the point of the story can be lost or become cloudy. Storylines become difficult to understand piece together because a loud part of the fandom wanted two characters to be together, despite it not canonly making much sense at all (and sometimes it is something that might make the actors feel awkward about it as well). I like to consider myself a loyalist to tv shows, loyalist meaning that I want to know the story the producers and writers had intended to tell when they had first proposed their series to their respected network. I don't tend to complain much about what happens in a show - as long as it all connects together.