I took spine as a d-elf AP a few months ago. I ended up switching to sword+shield for tanking every time. I was looking forward to increased defenses prior to taking the edge and I was _really_ disappointed afterwards, specifically because of lack of defense. Sword+shield was better, at least for that character (all relevant skills were maxed out).

Having written this, I'd still recommend spine. In my mind, progression for spell-caster APs is like so: sleep + vault kills up until respectable weapons, then switch to whip with lashes of the slave + mace off-hand for crippling strike after you can pump out some damage and have some HPs to spare.

Just don't pick spine thinking that you'll have decent defense boost.