If the builder didn't use mortar to hold the bricks together, he's going to be done faster, spending less time, than had he used it. So imagine if the wall was built correctly. We'll say that this represents 100% of the needed programming time. If I throw some crappy wall together quickly, most likely because my boss is saying, "Don't worry, you can come back and put the mortar in later once we are keeping the bears out", I've saved myself, say, 30% of those coding hours. Now the wall is falling apart, bears are all over the place, and someone is saying it was a poorly built wall with bugs. In reality, it was a poorly built wall on the cheap.
Of course now you have the job of sticking mortar into a built wall which sucks and is gonna cost you a lot more.
Best solution, fire that guy, hire someone else who is going to come in, tell you the last guy sucked and he's going to do it right and will just build another wall around the existing wall.