Some of these comments are basically saying anyone who drinks is an alcoholic.
A lot of good filtration is like a good haircut. No one can tell you've had one. If people are seeing portfolios full of nothing but black skies they are looking at a filter alcoholic. There are plenty of filter users that are not alcoholics. There is a lot of territory between cementing a red filter to your normal lens and being a teetotaler.
Something else that I find disturbing about filters is there seems to be a large number of people who don't know what they are for. Yes they can darken skies and increase contrast but they can also be used to cut through haze, remove glare, hide or accentuate skin blemishes and also change the relationship between colors/shades of gray. Digital shooters and even some color film shooters (surprisingly) wonder why I shoot black and white. Well there are multiple reasons including dirt cheap easy developing and enlarging. But one big draw for B&W film is the use of filters. It is something that can't be done in other mediums. Polarization is the exception. If I want to take a picture of a landscape and cut through haze I can use an orange filter, red filter, polarizer or combine. The result simply cannot be achieved without the filters. Also if I want to lighten some spring foliage while darkening the sky a bit I use a green filter. Again it really can't be done any other way. There is Photoshoping but the results are usually poorer (noise, posterization, strange digital artifacts) and of course no darkroom print .
The only thing I can say is people need to read about filters and then use them in various situations. You don't have to use a sledge hammer filter(s) in every situation. As markbau pointed out how dark your sky is also depends on geographic location/atmospheric conditions. I can take a picture of a landscape with an orange filter and have it look like an unfiltered picture taken in markbau's backyard. If my orange filter landscape is terrible then by definition everything marbau is taking in his geographic location is terrible at least in regards to how dark the sky is.
Seriously I am not a filter guru. As I stated before I will shoot a scene with multiple filters and sometimes no filter just to be on the safe side. If you are shooting roll film I suggest playing around with different subject matter. Filtration to me is just another tool in the workflow. I would not ignore shutter speed, aperture, focus, tripod, cable release, flash, type of film, type of developer, type of paper or paper developer. They are all tools that give you some degree of control. I have found when used appropriately filters can add a lot to a photograph. Heck even when used "inappropriately" you can end up with something nice and unique even if it is not very realistic. And just as I would not have a portfolio full of blurry, expired film, light leak, Holga images I would not have a portfolio full of black skies. Drink responsibly.