Argument from 'Responsibility' - One main argument is that, "I am charging my child rent to teach them responsibility." - This seems to be flawed, as any child can be taught responsibility without financial obligations from their parents. Sure, you can teach your child how to make a $300 payment every month to his/her parent, but what are the child's financial decisions every other day throughout the month? Are they blowing money on alcohol or materialistic things? Are they wasting money on entertainment and material instead of being practical? These are core values that are fed into our society through the media and other sources that are not going to be "fixed" by having your child pay rent. They should be learning how to keep a full time job, manage their finances (credit cards, car payment, etc.) and building their credit. From their actions, you can use this information to decide whether or not they need more help in becoming "responsible". But again, the help should not be to give them another financial obligation, as they would need to learn how to prioritize their prior finances first.
Your child is now your tenant - When you make your daughter/son pay rent, you start to create a gradual segregation. It may range from minimal to significant, depending on the child, but it does create a mental boundary (on some level) in the child's mind that he is starting to become separate from the family. This type of segregation may cause them to subconsciously act in ways that tend to be more selfish toward the family, as they feel like your tenant, and not your son/daughter. Examples: How much time they spend with you, how they treated you before and after the rent obligation began, the amount of money they spend on you (b-days, x-mas, etc.). Again, these may range from very subtle, to very drastic.
The Parental obligation to your child - When you have a child, you carry a moral and financial obligation to take care of her/him. Our society will state that, "Once your kid becomes 18, he's a grown adult. He needs to move out or start paying rent." - The first problem with this is drawing a line between the child's age and parental care. Apparently when they turn a specific age, the norm is to become less caring, rather than more caring? Why wouldn't care come in the form of making sure your child is financially and mentally ready to live on her/his own, rather than assign an age (18) to it? The second problem comes in with unrealistic and illogical standards for the times we live in today. As most people know, it is very expensive to live on your own. Many people I know personally, are moving away from California because it is too expensive. These are grown adults who have very stable and well paying jobs, and not even they cannot afford to live here. Now imagine your child who turns 18 and just graduated high school. Or if you want to push it out even further, imagine your child who graduated college, is now $50,000 in debt from school loans and may now have to move-out or pay rent from his family. Why not help your child, teach him financial responsibility by helping him build his credit, become frugal and/or practical for everyday purchases and if you can afford it, NOT obligate him/her to pay rent while they learn how to do all that.
A wiser investment than renting - As a quick note, when you don't obligate your child to pay rent, she/he has more money to save up for future investments (such as a house). If a child can live 3-5 years rent free, they could probably save enough money to put up a down-payment on a home.
When I was 18, my parents told me that if I went to college, I would live rent free. I went to college for less than a year and didn't like it, in which I then found a job and started to pay rent. I believe I paid $300/month, and this lasted from around 19 years old to 26 (when I moved out). This totals out to $25,200 ($300x12mox7yrs). Some people may look at this as a huge favor that my parents did for me, while others may say that I could have used that money to move out sooner. This is the entire reason of the post, and I welcome all comments/criticism.
Thanks for reading :)