Maybe some people think it makes them more personable? A desire to show personality beyond what shows in their shoots? I can see how a whole profile full of them could be a turn off but on the other hand some times these pictures are all models have in the beginning. Once a model has a solid base of pictures from shoots to choose from the casual shots can probably be phased out.

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Jokers
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Dona thinks Pie Bear is cool
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I'm an art photographer.  My goal is less commercial and much more artistic.  I look for a hook, a personality quirk or connection that I gives me a sense of what that model is like as a person.   

The 'non-professional' photos in question often give a much better sense of the model and how well we'll work together.

I had a photo of my in the bathroom of the local club with a sign behind me that said "Toilet Cam for Research Purposes Only". I thought is was funny. SO I posted it here for a bit to get a laugh or two. Apparently some people didn't think it was funny. It came down. And no one even read the sign! IT was funny, I tell you!

Don't worry. My Myspace colony got it.

I have pictures in my photography portfolio of me, and pictures in my modelling portfolio taken by me. But hey, that's kinda irrelevent I guess.

I've often wondered why models post cam phone pictures of them & their friends etc. too..