It is a definite disadvantage to play a rebuy without funds available, but it`s not insurmountable.
From my own notes, I see that I have twice this year won a rebuy outright without rebuying (although I did take the addon in both cases). I have also had a 3rd place where I took neither a rebuy nor an addon.
Those are the exceptions, though, and arose because that was the way the game developed. When I start a rebuy, I am always prepared to spend 3-5 buyins according to need, and I play $3, $5 or $10 rebuys, so the investment is fairly large.
Having said all that, if Pigpen is right about the structure, his point about EV is well made, and (as you`ll gather from my experience) it is not always necessary to spend more.