If you've got long-travel shocks under your hood, chances are they're attached to a shock hoop of some sort. Without a crossbrace to tie the shock hoops together, your shock hoops could be slowly killing your frame. Why? As your suspension cycles up and down, it puts a side-to-side load on your shock hoops. If the hoops can wiggle enough, they'll eventually tear the frame. The taller your shock hoops are, the more side-to-side (lateral) force they'll exert on your frame. If you're not crossbraced, you should be.
There's always an exception
The newest-generation trucks with factory-installed coilover buckets don't need crossbraces as badly as the others. That's because the factory coilover bucket is very low-profile and doesn't exert the same lateral forces on the frame that taller shocks do. Of course, if you're building one of these trucks for racing or general high-speed desert running, disregard this notion and brace away, tying the top of the coilover bucket in with the rest of your roll cage.