NARC seems like a great little technology to upgrade your reserves inexpensively. As front line tech, it's not great, but as a cheap upgrade it works fine.
Suppose you're the Star League, and you're leaving a garrison on planet Dirtball V. You stick a mere 3 regiments of battlemechs there (just a token force, really). As part of that force, you pull a battalion of Archer 2Rs out of mothballs. The garrison force isn't likely to face anything with too high a tech level -- the bandits and periphery kingdoms (and even most of the House forces in case they feel froggy) are all using 3025 stuff. So you put a company of Kintaros with the garrison, and maybe a company or two of vehicles carrying the NARC, and you just leave advanced ammo with them.
Boom. Suddenly your battalion of old tech mechs has a nice little edge, and you didn't even have to refit them. All it cost you was a company of medium mechs and now your fire support battalion can fire indirectly without a spotter, and gets better odds of missile hits. It's not an amazing improvement, but it's not bad at all for basically being free (yeah the ammo is more expensive, but you aren't having to rebuilt battalions of mechs).
When the Inner Sphere found it, it was basically just old Star League tech. It didn't really suit their modern needs, but they didn't know that at the time.