There are pros and cons to using live and training rounds with all weapons. In particular, since the 84mm requires the gunner to obtain experience with the sight's and firing the weapon to increase his likely-hood of hitting the target, it is beneficial and economical not to waste money on rounds that would mostl likely miss the target in the first place. Until gunners have a sufficient amount of time and experience on the weapon, it is pointless to expend expensive ammunition, just because he wants to hear a big boom. The training round for the 84mm has a tracer in it, thus you can observe strike and determine wether or not you have actually hit and used your sight's correctly, being extremely important, since further targets are harder to hit.
Having sufficient experience as a Sgt and pointy head, understanding this aspect of training with sub-calibre rounds for weapon systems is beneficial and only produces confident and experienced gunners that will be able to kill targets when asked to do so.
Many military's utilize training devices for their weapon systems such as lasers and small calibre rounds, and with the army getting more and more technical, one should accept these systems and grasp what he/she can learn from them.
