Is the concentration skill necessary for any of the shifter abilities?
I would say no, but I haven't played shifter recently. CapnQ will know better.
It *is* possible for shifting to be interrupted, so you might want Concentration for that reason. However, it's quite rare for it to happen, and you can always just shift again, so if you need the points elsewhere then put them elsewhere.
It's also worth remembering that Concentration can be useful to a Shifter if he casts other spells. For instance, in one of my favorite builds, Shaman / Shifter / Shining Blade of Heironeous, I take Concentration to prevent Divine Might/Shield, Shock/Holy/Brilliant Blade, and my Shaman spells (mostly Restoration and the like, as I don't have enough caster levels for offensive spells to be very useful) from being interrupted.
