Well, I see both sides to this. My son (2 1/2) has never slept in our bed. He's been in his crib since he was home from the hospital (spent a week in the bassinet with a bili-blanket on for jaundice). My bed is too high for him to climb on, so that helps. Both my brother (now 25) and my youngest sister (now 15) slept with my parents and it wasn't a habit that was easy to break! My mom actually ended up getting a full size bed in my sisters room so she (my mom) could actually get some sleep in there. SO, after seeing that, I wouldn't encourage my son sleeping with, much less ON me. Lol.
That being said, just the other night, I said to my husband after I went in and checked on our older son, "I see him there and I just want to go over and cuddle him all night." He has a toddler bed. Then, the very next night he woke up with a nightmare and I did lay down with him. It was awesome to have him snuggle with me for comfort. I haven't done it since as I don't want to start a habit, but anytime you can get some cuddles from an active toddler, it's a bonus. If it's not that often and you (AND hubby) are ok with him being in your bed. Let him fall asleep on you and move him back in his own bed or off of you.