if oscillator stops in sleep mode there is no point using a WDT
Just imagine that due malfunction or even bad programming your software jumps on instruction which invokes power-down mode (even worse: there is no such instruction in your code but just bytes sequence in DB does same). If WDT is still worked it could generate an reset. Unfortunately, Atmel`s WDT stops as well -> your software hang ups in the whole with no chance to be rebooted.
Good days!