There is certainly an interesting debate here.
Surely the one thing we can agree on is that mountain biking is mainly about having fun and some folk have a different idea of fun to others. In the fortunate position of having several bikes I have found an XC full suspension bike will get me around most trails effectively, seldom wallowing into suspension sag hell and generally keeping me awake enough to tackle most thing from the reds at Bike Park Wales, to stage races in Canada.
Hardtails also have a place be it racing XC, cruising the local trails or out backpacking, an XC hardtail can be too little bike on some trails. Sure some folk will get away with it but personally it is not for me anymore. I have previously got one down the red run at Fort William and around many of the trail centres in Scotland without mishap, but for me a full suspension option is more comfortable and quicker in almost all situations.
Longer travel full suspension bikes (Trail to Enduro) have got to be a good idea for the steeper, more technical end of the spectrum and if that is what you enjoy riding then why not get one. It will do the easier trails too – just no so fast. And there is nothing wrong with this.
So, IMHO it is possible to have a bike that has more travel than necessary for the trails you ride, most of the time an XC full suspension or hardtail bike will see you through. BUT there is nothing wrong with riding something with more travel on tamer trails. It just won’t be as quick on the climbs and may make the descents less interesting/exciting that they would be on a cyclocross bike.