Damfy(?) at Damfacrats has a dandy ten item list he(?) calls "ten motives for amnesty stunt". I found these two particularly interesting:

4. Make Bush pundits look objective by allowing them to criticize Bush about something. All year long we will hear, "[You can believe me because] I don't just attack Democrats, I don't kiss the Executive Branch's feet, why, I spoke out strongly when the Commander-in-Chief wanted to let those illegals stay."

5. Document immigrants so DHS can efficiently deport them when some hysteria arrives.

time will tell-- although if I had cable I'd bet I could already tell you about no. 4.