Im trying to get my head around if there is an actual advantage to having a vehicle with full time 4wd (for example, a Landcruiser wagon) as opposed to a rear wheel drive car.

My understanding is that most of these "permanent 4x4" vehicles have a completely open centre diff until locked (talking more about offroad vehicles rather than sports cars which probably have a centre vicsous diff etc) and therefore still can only drive one wheel at any given time.

Can someone please explain the advantage here? is it because under certain circumtances it will maybe drive a front wheel and promote understeer?

Take the mistubishi super-select 4x4 system for example. It's claim to fame is that you can drive in both 2x4 or 4x4 on the tarmac (with the diff unlocked only). Without that diff locked, and therefore completely open, how is the 4x4 mode any better on road than the 2x4 mode?