What's the point?

Strange move by the Conservatives, who are promising to pass a law prohibiting any rise in Income tax, VAT or National Insurance over the next parliament.  This of course is if they form the next government - but why?  Firstly as has been said before, the basic constitutional position is that parliament cannot bind itself - anything parliament can do, parliament can subsequently undo, so does it really mean anything?  And then why paint yourself into a corner?  Suppose we have another Lehman Brothers situation coming out of the blue - if you can't raise taxes and you are opposed in principle to borrowing, how are you going to deal with that?  Smacks a bit of meaningless posturing I think.