You have 2 essential factors that will need balancing up here:
1. The airframes of the 3 shuttles are getting old, and will need a lot of attention and inspection to ensure safe flights for a further 5 years. I believe all 3 existing airframes are out of certification when they finish the currently planned mission run (likely to end 2011). That means all will need a complete refurbishment to be flight-worthy beyond that. Discovery, being the oldest airframe will likely be retired and vital parts used to maintain Atlantis and Endeavour. The proposal also I think suggests shutting down to 1-3 launches a year. This infrequent use means even more safety issues.
2. The US really need to keep sending people into space and maintain it's launch capability, and ability to send it's astronauts to the international Space Station.
Take a view where you like, but like ssjfb says the last word lies with President Obama.
