Paying clients dont get water marks on them.
TFs do because your promoting the model as well as yourself.
WIth out Watermarks you would run of the risk of not getting promoted and then why did you shot free.
Two If you didnt get paid and they use your image for something. WIth out watermark its harder to show you only
meant it for TF use only in court. But if you had a water mark and they removed it. They cant say they didnt know.
It helps all around. Some Photographers watermarks are really cool and add to the image! Wish mine was so cool!
If they dont want watermarks just charge them XYZ Because now it became a paid shoot!

I'm one who always watermarks my work unless a client is paying for the images.
I've had the misfortune of finding my images having been stolen and used/abused on dating and escort sites, some even with a watermark intact.
Of course there are instances where models innocently forget to list credits which is easily fixed, but I'd prefer not having to chase things up like this if we both worked for time and prints.
As mentioned by Yani, if two parties put into a project both parties should be promoted. 

...I'm not even going to get into models using TF images on their Patreon accounts.

I have found a great solution is to send the clients unwatermarked images & a bonus smaller watermarked image for social media.

Many of my photography clients love to post the watermarked ones, & it helps marketing for new clients.

I stopped using a watermark (https://www.modelmayhem.com/portfolio/pic/28818279). I noticed that the better models I worked with would crop out the mark. Eventually, I learned to hate it myself. There also came a point in which I decided my work was recognizable enough. I like the clean image.

In my opinion watermarks should not detract from the image. Models and others who object to watermarks are often on those images that are large, solid colored, and placed over the subject so that it detracts from the appeal of the image. Of course, if it is a proof image, with the expectation that a client will purchase the non-watermarked image(s), this could be a different matter altogether.
A watermark, if removed by anyone, may have value if a case appears in court. If a photographer regularly regularly watermarks his/her images and an individual removes the watermark because he/she 'did not like it' there may be a case of willful infringement of copyright.

The worst watermark I ever saw had been photoshopped onto the model's body to make it look like a tattoo. Totally obnoxious, but I'm sure the photographer who did it thought it was awesome.

I only watermark images on the web.  It's so easy to just create a PSD file with your name and copyright (© 2017 Rick Davis) and then copy and paste it on your photo.  You can then adjust the opacity to make it barely visible.

If a model is wanting to make prints, or I'm shooting models for an agency that I know will go in the model's book, I don't put a watermark on it.  There's no reason to.

You can usually tell how new a photographer is by the size of their logo/watermark.