Really the question for this should be: Do you want your toddler wandering around the house while you're messing with a baby and do you want a safe place to put him when you're attending to the little one?
There are amazing toddlers who will obey you at 18 months and stay in their rooms like little angels. (I only know one mom with a child like this.) There are other ways to ensure your older child is safe from harm while you change a diaper. But don't pass up the crib if you don't need to.
When I found out I was pregnant with my second, I realized no way would my first be ready for a bed--he would be 16 months when she was born. I bought a crib second hand (actually I think it was third hand) from another Baby Bunching mom who had just moved her oldest to a big boy bed. It was a smaller crib that would work up until my daughter was only about a year. Sure enough when my son turned two, he wanted a big bed and she was ready to move up to his crib. We sold the crib again to yet another Baby Bunching mom doing the same thing.
The crib serves as more than a bed. To oust your oldest before he's ready (or you're ready) can unleash all kinds of other issues you may not be ready for such as room destruction and disappearing naps. The latter is crucial. You want to keep those naps as long as possible.
A few things to keep in mind:
1. The size of your house. If you live in a Manhattan apartment, a second crib may be a space issue and your oldest may have to move to something else or the baby will need to be the Pack n' Play longer than you anticipated. They can last in there for quite a long time, surprisingly.
2. Budget. You may not have money for a second crib. If you think you need one, you'd be surprised how many other moms would gladly free up their attic/basement while you borrow one for a bit. I have one sitting here if someone needs one.
3. Your child's temperament. If you kid seems ready for a bed and she's a well-behaved 22 months old, I'd say go for it. If you have a rambunctious 17-month old that likes to ransack his room, maybe not quite yet.
4. Spacing. If your kids are closer to 13 months apart, clearly you'll need place to put your new baby. If your kids are closer to the 24-month mark, you could fudge it if you need to.
But our recommendation: Keep the crib as long as possible.