I forgot one more thing... to get hired on as an artist somewhere, a person needs to have a well developed portfolio. Your portfolio will get reviewed by potential employers, among many others. They are judged on several levels. Undestanding of artistic elemennts like lighting, composition, color, communicating a message, and creativity, originality and technical ability within the applications.

On has to admit that use of pre-made content is not going to get you really high marks on creativity, originality and at the minimum cast a shadow of doubt on technical ability.

About various CG sites, a person does encounter a lot of young artists whom are still trying to make it in the field, and whom are in the process of trying to get noticed and building their portfolios, whom are all to conscious of all of the above. 

Also, it has a lot to do with copyrighting too. Poser content, of course is made to be used and expanded on... Lot of times when you are hired to do a piece, the paying customer wants 'exclusive rights'. Well, with poser content, it would be pretty hard to create a very exclusive piece that didn't carry a strong resemblance of another character already in existance. I'd have to review the license agreements for Poser content, and see if you are able to create a piece using it, and then sell exclusive rights to it. 
What's to prevent someone from taking, I dunno, Aery souls Alice, make a portrait with default pose, clothing and ligting, and sell it to someone unsuspecting claiming they have exclusive rights to use that image, for a few grand. I would think that has a potential to be a big can od worms.

You could, by making your own textures and making your own morphs etc... then you could do a final render and scene assembly in poser.  I can tell you, having the ability to do the above, model, UV map, texture, assemble a scene (at lest technically), and knowing several other apps, Poser is the last app I would switch to from the modelling aps to put a scene together. Comparing few other applications I use, it's a pain to use Poser, nor could I do some things in a timely manner in Poser, if I were on a deadline. For one, compare setting up a magnet and adjusting a piece with a magnet, to being able to select a few vertices and deform a mesh in other apps. After a while, if you're spending double or three times the amount of time it takes to create something, you're watering down your income as a professional, to say the least.

The times I do use poser is when I'm trying to prepare content for someone else to use in poser.

Anyway, that's another consideration for professionals, since they often sell exclusive rights to their work.
Also, what if someone wanted to contract you to give them your meshes too? As is often the case in some of the work I do? Poser content license agreement doesn't allow you to redistribute them. 

See, how using Poser and pre-made content can a number of limitations? Lot of times it's not just the look of the end product that matters.

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