Yes, that is a concern, but I think that's a concern with any sort of recharge mechanic.
I basically just told my group that if they try to take extended rests too often (like after every 2 encounters) then they will get attacked while resting.
Plus, I am hoping that this mechanic will increase the number of encounters between extended rests since the PCs will have a way to recharge their daily powers. Hopefully this will mean that the PCs will take an extended rest when they run out of healing surges and not when they run out of daylies (which is what was happening up until this point. My PCs would have half their healing surges left, but would want to take an extended rest because they had used all their dailies.)
And, truthfully, my PCs were already a little too close to the "going nova" end of the spectrum anyway. At least this way the PCs can "go nova" and then continue without an extended rest.
