Alright, since you guys seem to be having trouble with this whole analogy, I'll spell it out for you:
Roots: All radical Muslims in the Middle East convoluting the thoughts of otherwise good people into committing terrorist attacks in the Middle East and US, or anywhere else.
'Top': The people who leave the Middle East and try to commit terror attacks. The reasoning is: If we take them out, great, one less terror attack. Unfortunately people will keep coming from the Middle East to try and kill us (yes, you and me, regardless of what we think about the war.).
So, considering that Sadaam wasn't one of these radical Muslims, nor was he committing terrorist attacks against the US, can you explain to me again why Iraq was, as you called it, "fair gain"? And I don't mean the WMD bit, because the reason you gave was specifically the roots/top bit.