If the Republicans go for it all at once, the political PR will be that they want to starve the poor and the elderly, and this will hurt the cause in the long run. By dealing with it piece by piece, while very frustrating and very slow, progress is being made, because the dems cannot make a case to keep specific proposed cuts in the budget. At some time, the big ticket items will have to be hit, but now is not the time.
Boehner is winning this fight. Remember, he's currently winning cuts on the DEMOCRATIC budget (this is the one the Dems were supposed to pass in 2010).
The first real Repub budget (for 2012) hasn't even been discussed yet...that is where the Repubs will get much more of what they want.