Models are used for several different kinds of things.

For instance, if you're a photographer looking to build your portfolio, and you have models looking to do the same, then trade is fair. A model will take photos for his/her portfolio in exchange for modeling.

Modeling isn't the easiest task in the world. Sometimes it's long hours, weird, uncomfortable positions and sometimes risque poses that not just anyone would do.. especially for free lol.

With me, I occassionally use models for my art. I can sometimes put them through alot lol. But if I expect them to sign a release so I can sell my images, they're going to want compensation as well.
 

You could use the same argument for families paying photographers. If you are working on a personal project and don't want to pay models.....you have 2 choices when it comes to TFP or Time for Photographs......you get inexperienced models looking to build a book , or you get experienced but out of work models working on their books. If you have a specific job and need a specific kind of model with experience you are going to have to pay. If not you take your chances.
 

And, just to be fair...not ALL models need to be paid a lot of money. I know of cases where models will sign a model release by being compensated $15/hr. This isn't an unreasonable price for photos that you would try to market and sell.

Also, when you hire a model, you can expect that they'll know how to handle themselves in front of your lens. You should, conceivably, get more useable photos in a shoot with a hired model than you would with someone just starting out doing TFP/TFCD work.

Just my 2 cents...
 

and adding to all of the above, it is difficult getting models to even show up in the first place. Compensation tends to help with that problem.