The use of ornithine is to help with ridding the brain of ammonia as a result of various parasitic wastes. There is nothing actually requiring you to use ornithine; I've never used it, and I've never had problems sleeping (but then I never have).
If you have problems sleeping, ornithine is good as it helps mop up ammonia which can keep you up at night. If your sleeping problem is caused by something else, then ornithine (probably) won't help. If you don't have a sleeping problem, and you don't have much ammonia buildup, then you're probably OK to skip it altogether. If you like other remedies, that's fine; again, the ornithine is specifically to help handle ammonia buildup in the brain, which can cause sleeplessness.
If your urine smells of ammonia, you would do well to take ornithine for awhile.