Here's my reason---when losing one house temporarily forces our party back to its Conservative values (less government, less debt, less spending)--we (real Conservatives) place ourselves in excellent position to hold power for a very long time.
Now, if we hold both houses and the executive branch for the next two years, we are going to have to answer to voters for what many of them PERCEIVE as some consistent mismanagement of our country. Make the political calculation here. If one house has a dims majority at that time, it will be very easy to paint dims as responsible for it all.
I'm not swallowing all the polls here...but I don't want us to be so naive as to miss the end goal: not merely two years of government by a group who are only interested in acting like real Conservatives sometimes to appease us...but in longterm government by true Conservatives who would not forsake their base on spending, immigration, etc etc.