So Skaven Slaves are 2 points per model. That's because they suck, right? Wrong! Well... not exactly wrong but it's all in how you use it. 2 points per model, because I never upgrade my Slaves. Shields offer longevity and survivability, but I don't care to add points to an expendable unit. Same reason I don't use spears or slings, as well as the fact that Slaves are not there to kill anything (and rarely will). They have some of the poorest stats in the Warhammer world other than an excellent Initiative 4 which will sometimes beat an opponent to the punch and take away a few attacks.
So how do I use my Slaves? As giant tarpit units. Combined with the amazing Strength-in-Numbers Skaven army rule, if your Slaves are within range of your LD7 General they turn into LD10 Slaves with at least 3 ranks. I run my Slaves generally in 2 units of 40 models, ranked 5 wide and 8 deep. This will keep me Steadfast for longer at LD10 and hopefully with a BSB reroll. BTW that unit of 40 models with a Musician (only) is 82 points. It is a STEAL.
The Slaves are at the front of my army, ready to be sacrificed as any good Skaven Slave would do. I run them towards my enemy and either slam into elite units hoping to hold them in place, or park in front hoping to GET charged and save a round of combat. Many of my opponents know what the plan is and charge them anyways with nowhere to go. Otherwise they waste valuable game time trying to maneuver around my large blocks and may end up getting charged themselves later (in the flank).
As you set this up it is important to protect their flanks to keep their Steadfast LD10 intact. They remain Steadfast if flank charged, but it is the +3LD from Strength-in-Numbers I am interested in retaining. This is why I often deploy my Slaves near the center of the board with my support around them. Often in the form of Storm Vermin, Clan Rats, and General / BSB. I am particularly a fan of getting the Slaves stuck in, and then next turn charging my Hell Pit Abomination into the fray. The Slaves remove enemy Steadfast and the normal problem a HPA faces. The same with a Plague Furnace.
Slaves hold up a unit of Grave Guard with a HPA looming in the flank.
Also, I often field my Skaven Slaves with a 15 point Warlock or even one of my item carrying Warlocks parked in the unit. This will keep them at LD8 with SiN at the very start of the game if they are out of range of the General (which they should never be, but oh well). It also acts as a delivery unit of your Doomrocket or Brass Orb, and then as a further redirector if you use that lone Warlock correctly... but that is another tactics article.
So remember my fellow Skaven Warlords, it is worth it to bring the large units of weaklings. Spent your Warpstone on as many Slaves as you can get!