This season Pep wanted Abi to multitask and to my understanding, the frenchman has performed extraordinary well on both LB and also on CB when required.
With Piqué & Puyol are on the team, clearly he's got to be the LB to keep the strongest 4 players in the back (with Alves).
However, if one of the CB's is missing, then we have a solid debate: Secure the LB or give strengh to the heart of the defense?
I'd like to think it depends on who we're playing against.
For example, wihout Puyol and against Arsenal, I'd keep Abidal as LB to keep Arsenal's wingers in check.
But... if the opponents were Chelsea, I'd place Abi as CB and Maxwell (or Adriano) on the LB.