Different strokes for different folks. I know some psychologists say it's a no-no. Besides the potential for some type of intentional (or unintentional) sexual "awakening," there is also a concern that a child often allowed in Mom and Dad's bed will always be wanting in there.
My take? It's okay for children to hop into bed with Mom and Dad once in awhile, (bad dreams, unusual circumstances, etc.) but probably best to sleep separately as a general rule.
In the last year, in the process of moving with my (now 18 year old) daughter, there was one time when a hotel had no two-bed rooms available--so we slept together in a King Size bed. And once she had to sleep in my personal King-Size bed--when hers had already been packed. There was no discomfort whatsoever, but I can't imagine a scenario in which a mother, father and 18 year old would, for any extended period, decide to sleep together.