This shadow isn't designed to swarm. Southron Runner adds (2), while costing (1). The idea is indeed to generate pool for the skirmish phases and do the damage there. The 2 strength of Spear are enough imo if they can't exert and they don't make them fierce. Which would either kill them completely or too strong. 2 Mumak are fine I think.
If it's not designed to swarm, then is designed to beatdown, or to corrupt... or to lose. It's not bad designed at all, but those small guys can't win skirimishes nor exert with
Southron Spear (or
Flanking Attack). They are quite susceptible to direct wounding or arrows... I mean, a 6 strength guy with Spear and
Mumak, a 11 strength fierce ambush
![2 [2]](https://lotrtcgdb.com/forums/Smileys/classic/2.png)
southron is an easy target for a
Quick As May Be or a
Ranger's Bow if has only 1 vitality. Those card slots are worth of better minions/items/pumps... shouldn't be considered minions at all but twilight adding events if they remain, don't count them in the minion ratio just as Bill Ferny, Saruman and
Wormtongue can't be counted that way in an Uruk deck.
Southron Traveler is a great minion, don't put him in the same bag...
Those small guys can't do the beatdown job, but a
Mumak or a Spear can. And some surprise twilight for an equally surprising
Whirling Strike can too...
No one has made a mention of
Flanking Attack. The difference between Ambush
![X [X]](https://lotrtcgdb.com/forums/Smileys/classic/x.png)
and a
Flanking Attack is the surprise factor, just as the difference between a
Goblin Scimitar (or
Guard Commander) and
Drums in the Deep... Thus it can turn a safe predictable assignment into a perilous gamble. And do the beatdown.
I kind of saw him that way too. He's like a Goblin Runner with more strings attached. He can effectively add 1 to the pool for skirmish punishment 
Goblin Runner can swarm very well. And adds twilight in the Shadow phase. Those are quite different minions in quite different cultures and dynamics. You can even add 3x
Goblin Runner instead, as it doesn't have to survive maneuver/archery, is exactly as predictable and allows the twilight in the Shadow phase...