The notion of water conditioners reducing scale is one which is probably destined to be debated forever. In the interest of full disclosure, I am in the field of mechanical water filtration, including water softening. That being said, for my entire time in this field (13 years), I have been searching for the answer.
None of the manufacturers of these systems will submit their product for testing to verify their claims, so there is no evidence, other than anecdotal, to prove their claims.
In my experience, the only way to prevent and even reduce existing scale in the system is with a water softener. Water softeners have come a long way in regard to efficiency. Salt and water usage has been greatly reduced in the past 10 years to the point that since most systems only regenerate when necessary (based on the consumer's water usage), users are replacing salt between 8 and 12 months.
With a water softener, you will not only prevent and remove scale, but you will also realize the benefits of soft water throughout the home, such as smoother skin, more manageable hair, no residue on dishes, whiter clothes, less lint in the dryer (which means longer-lasting clothes), and even longer life on water-using appliances. These are all things which you will not see from a water conditioner as they do not remove any minerals from the water.
Speaking of minerals in the water, if you are concerned that you will be losing out on minerals in your drinking water, the amount of minerals in hard water is minimal; most come from your daily diet. If sodium in the water is a concern, it is important to remember that the amount of sodium added to the water is exactly equal to the amount of hardness removed.
Finally, if it's sodium you are looking to avoid, any water softener can use potassium salt as an alternative.
For more information:
To learn more about healthy water, read Tammy Schwolsky's Ask A Pro Q&A, "Can you recommend a whole-house water filtration system?"