you'll need adapters because the bolt patterns won't match, and you'll be carrying a bunch of excess weight putting more stress on your unit bearings. Also drives you to MUCH more expensive tire options than you'd find with a smaller wheel.
As said, you can do it with wheel adapters because of the different wheel bolt pattern. I see no pros actually. You need wheel adapters, replacement tires cost more, and 18" wheels look lame on a TJ.