I think that being able to invite family or friends to come along and stay for free is great. As a matter of fact, our first stay is in April and we invited some of my wife's family to come along. We don't expect them to pay anything for the room. However, knowing my father-in-law, he'll probably try to pay for most of the meals for all of us.
However, I think that there may be situations where people would be sacrificing a vacation, or several days of one, by having someone stay with them because they need to get a larger unit. So, in this case, I think you need to determine what you feel comfortable asking these people to pay, if anything at all. Just make it clear up front.
In most cases, people would be getting a better deal by staying with you and contributing some money than they would by staying on their own.
Also, I see DVC as slightly different than owning a vacation property or inviting someone to stay at your home. With DVC, most people would be using more points if they brought along guests. Inviting someone to stay at your vacation property doesn't cost you anything.
If you have the points and want to have people stay with you for free, that's great for everyone involved. However, if you can't "afford" the points then either don't invite them or make it clear up front what you would want them to contribute to stay with you. This can be great for everyone involved too.