Several points to make, not all of them germane or pertinent.
Forgive my rambling...
I've heard many reasons from senior leadership over the years:
A - XX% take the bonus, so it must work
(this does not mean that there are those on the fence who may be influenced by the money, but I have to believe this is a small percentage overall)
B - It's really meant not as an incentive to stay, necessarily, but rather a reward for staying.
(The logic here is that there are other career field bonuses (medical and legal spring to mind immediately) to compensate folks for some portion of what they would receive in a civilian equivalent position
C - It is to reduce the flexibility of the fliers
(This speaks to those who are talking about the 7-day opt out)
Let's face it, AF Manpower is reactionary and not proactive. They are just dealing with the whip end of the tail. Manpower folks talk of red line/blue line and bathtubs and think these programs have significant impact.
Truthfully, in addition to what I said earlier about those who stay are going to stay anyway and those who punch are going to punch anyway, I think the economy and outside hiring have far more influence on folks' decisions than the total dollar figure of any bonus...
All that said. I do like what the RAF does with Specialist Aircrew. A bonus system rigged around something like what they've got would've been nice. Without getting into details of their system (because we don't have it an likely never will, so why bother), suffice to say their retention seems to be as good as ours when it needs to be and as bad as ours when the majors are hiring...