Personally,  I  would  use  it  as  a  book.  But  I'm  also  not  the  brightest.  I  see  no  real  reason  not  too  though.  

It depends how far into the game are you. I've seen that a double Steelskull in Faction Wars can carry you a lot with the double cleanse and heals. If you're not bothered by FW or have other better champions, just use it as a book

As senda Said Double steelskull is almost a must for the last Skinwalkers FW boss. besides that no real use afaik.

Skinwalkers is a b!tch, I would save in vault until you are focused on 

 Ill  pack him away for later

One Steelskull should be enough unless you dont have good epics all any lego from the faction, you can use a 2nd one as lvl 50 till you finish FW