The point of the NARC isn't to land extra missiles from the Stalker. It's to keep people away from the Stalker, lest every single one of the Stalker's buddies land missiles on it. Would you consider 5 tons a good deal if it meant nobody came close to you, lest they face the wrath of an entire company, or more?
No.
Because it requires me to carry five tons of dead weight for a threat I may never use. Plus the extra expense of NARC homing missiles that might never have a beacon to deploy. AND the bubble is nine hexes on something that isn't closing fast.
If we are talking company combat, the NARC goes on a fast light or medium that can close quickly, generate high TMM and actually let me use the NARC I'm paying the mass for. NARC is not a dueling weapon, it is a team weapon. As such it should never be mounting on big slow missile boats, you put it on a scout/spotter and let the rest of the company carry the missile launchers.
In that case, letting something small, and to be frank you are willing to sacrifice mount the NARC and place the beacons is a more logical decision than stuffing it into the slow ass assault boat. THAT you load up for maximum pain, both firepower and protection to extend the pain.
That five tons is a second ER Large Laser, 50% more LRM tubes and CASE, AMS, CASE, and more armor, or plenty of other things that are direct combat enhancements when mounted on the mech. As you point out you do not have to mount a NARC on a unit to take advantage of it. Putting it where it would be most effective is more threatening than putting it on a big slow unit that realistically will seldom get to employ it.
The AC/20 gets a bubble of doom because of the potential to covert a pristine section to breached, or destroyed, in a single hit. The SRM Carrier gets the same effect by toting enough short range firepower to shred a light mech in one salvo. Notice the instantly lethal potential and lack of a defense other than not being hit?
The NARC fails both, you effecively have a one turn delay, the bonus does not kick in till the turn after the NARC beacon lands. And if you have an ECM suite on the target? Not doing you much good while that is running. The bonus also only increases the damage, it does not boost chances to hit. So an already illusive target might risk it. There are simply too many 'ifs' for the NARC to be a 'bubble threat' weapon.