Havel's words are a beautiful expression of the power of hope. However, hope need not lie at the root of Walzer's imperative.
Hsieh's criticism rings true if we face a simple, one-period scenario in which we face two choices -- an "appropriate" one and an "inappropriate" one -- in response to a given conflict between the West and an Islamic world where militant forces have achieved broad influence. Walzer's strategy makes sense in a dynamic, multi-period scenario where our choice about how to respond will affect the future choices made by the Islamic world.
Militant forces may have achieved broad influence in the Islamic world, but they clearly have not achieved absolute control. By choosing to hold the door open for peace, we appeal to the interests of moderate and peace-minded muslims, encouraging the future to develop such that these groups come to resist the militant forces. The alternative, where we assume this simple conflict between two irrevocably opposed parties, sends a message to the moderates that there is no point in struggling against the militants.
Thus our "hope" creates a different future. This is not Havel's sense. From your except it would appear that he was referring to situations where there was no external reason to have hope, whereas in this case we can clearly influence and possibly brighten our future.
That's another good way to unpack "hope is for the hopeless." Of course, if things were incontrovertably hopeless, nothing would help. I think the underlying point is that we have to understand our own limitations as far as diagnosing hopelessness is concerned. Seems to me Walzer and Havel take up hope by way of humility: things seem hopeless, but if we keep acting, we may turn out to have been wrong about the hopelessness and things may change.
I have another perspective to add to the "hope for the hopeless" scenario.
If I am in an apparently hopeless situation, I think to myself: "I have two options: assume I'm right, be hopeless, give up; or allow for the possibility that I am wrong, that there is room for hope, and choose to hope." If I choose hope in vain, so what? What is at stake but pride?
I looked everywhere for another Havel quote where he makes much the same point: it was quite likely that nothing anyone did would change their situation, but actually doing nothing guaranteed the outcome they dreaded, and acting with hope was the only alternative.