Further, open code means that other researchers can come along and write unit tests. It amazes me how much code is still verified manually if at all in the sciences.
and that brings us back to why many scientists don't publish at all now...
As long as not publishing code is standard, there's a incentive to do it since it might open you to career-withering criticism...
I have tried to explain (With limited success) to my colleagues in bioinformatics that not unit-testing your code is like using an instrument that hasn't been calibrated, and blindly trusting the results to be right.
This is the standard procedure for when you've developed a complex algorithm to do something novel, and actually working through it by hand would take hours.
I should know, this is how I used to do it before I knew better, and as a result one of my published papers has (very slight) numerical errors in.