My son, just turned 3 this month, is still in a crib. The only reason we might decide to take the side off is because we told him if he slept without a binky, he could have a big boy bed (I know, a little late on the binky, but he hasn't had it now in 4 days).
That being said, I think he would still be happy in his crib if we decided not to. Also, when we travel, I plan on still having him in pack N plays until he climbs out.
I know sometimes, you think it would be so cute to have them sleep in a toddler bed, but you may regret it later. There is really no going back once you take the side off.
It is comforting to me knowing that he is safe in his crib at night.