If things are so simple as you say why this hot discussion and most of all why marinas or states do not want liveaboards?
Because people don't like it when other people live in a way that's different than them.
When person A sees people B-X making the same choices as them it validates that their own choice was correct. However when they see people Y-Z making a different choice, well, by God they must be doing something wrong.
You'll find this applies to most areas in life and you'll also find that some people spend a huge amount of energy and effort making sure that choices that aren't their choices are eliminated as potential choices for people who don't have their level of "enlightenment and wisdom".
Also you could argue as a race we're very much designed to "fall into line" and do as everyone else does. The tribes that acted like this more easily organized and wiped out the tribes that didn't. If you don't fall into your tribe's expected way of living and acting, don't get all surprised when the tribe takes a disliking to you.
It's rather absurd to think that liveaboards are taking more from a community than they pay in. We pay commercial rates in electricity, pay taxes/environmental charges in our slip fees and use far far less resources than your average house dweller. Hell, a house's lawn alone will burn up 10,000 gallons of water a year. I probably use a quarter of that every year for all of my needs. Never mind all the fertilizer that gets dumped on that lawn and runs off into the local waterways. And then people bitch about boat bottom paint or grey water discharge???
Don't look for any deep logic in dislike of liveaboards.