Years ago, my parents figured out the advantages to including friends. There's company for the child, so adults can still have fun with the kids without having to be playmates. This is especially important if you have an only child or siblings with differences in age, developmental levels, or interests. In our case, we thought, "Someone else to play Twenty Questions might not be a bad idea."
There are more complicated considerations, however:
*With busy schedules, some families don't connect much outside of vacations. Do you want to share this time?
*Can you afford the potential additional expense?
*Is there physical space for another person (in the car or at the hotel, for instance)?
*How will another guest affect personal privacy?
*Do you want daily responsibility for someone else's child?
*How will an extra individual affect family dynamics?
In our case, since we do have a lot of family time, accepting another party member was simple. The challenge was overcoming the other reservations. The solution? Inviting the right friend.