I'm having the hardest time to figure out what the negative is in this. Maybe it's because I don't plan on swapping carriers every couple months to care about 2 years of service.
I'd gladly take $450 over $140 any day -- if you got that in a gift card over monthly credits, even better honestly. Verizon let's you pay off agreements early so you could take that $450 gift card towards that. Another advantage with that over monthly credits is you don't lose any credits for an early buy out!
Even $20 spread out over 2 years isn't a bad deal. Do you honestly see yourself upgrading or switching in that time?