I really don't see how the existence of not of a secondary project should impact the decision about whether to play a binary Phase III or not. Both the up-move and the down-move will be smaller if some value is ascribed to the remaining pipeline - but you can adjust for that by increasing or decreasing your bet.
Yes, having a secondary project that the market ascribes some legitimate value to will at least help raise the floor in event of failure of a lead asset. And, you're right, you can adjust for a one-trick pony by just placing a much smaller bet (or by hedging). I just tend to be a bit more conservative and won't invest any amount where I think there is a legitimate chance of significant loss tied to one near-term event.