I agree. It disrupts the workflow, involves extra clicks to workaround, and if not careful can easily lead to incorrect dimensions.
I would like to permanently disable that function.
It seems that there's some new feature with every new SW version that people don't like and can't turn off. The dimension palette, note combining, and hiding components with the Tab key are a few other examples from the last few years. (Although one or two of those were fixed with later releases.) It's great that the people at SW are adding new features, but they need to realize that there will be some that people won't like and GIVE AN OPTION TO TURN THEM OFF.
This problem is "fixed" by moving the OK, Cancel, etc. toolbar so it is now above the numeric input field. This change is enabled by default in 2013 sp5 and SW2014 which will be released in the near future. In 2013 SP4, you can also get the new UI by using a special registry key. If set, the behavior would show the Dimension dialog with the OK/Cancel/etc toolbar along the top edge so it cannot be obscured by the drop-down box which offers other recent inputs, formulas, globals, etc.
You can enable the changes in SW2013 SP4 by setting the following registry key:
'HKCU/Software/SolidWorks/SolidWorks 2013/General/Dimension Dialog Toolbar Above' = 1
Let me know if this works out for you.