The most obvious reason is that the Finish is the only point on the map which does not have to be on a mapped feature and thus does not have a control decription.I have never understood the reasoning for this ("Navigate to Finish" should
not be used).
This makes it different from every other marked point on the map and means that it is unfair to ask people to navigate to it (what is "it"? What am I looking for?). Of course, in many cases, the Finish point is clearly visible and it doesn't matter if it's not a mapped feature, but there are many instances of the Finish just being an anonymous flagged stake in low visibility terrain leading to potential time loss in finding it.
There are other instances of finishers milling around and hiding the Finish flag from view, etc. which would not be a problem with a teped run-in.
So, I can think of many reasons why you should not have to "navigate to Finish" and only one potential disadvantage, that mentioned by King Penguin regarding a narrow/rough run-in. But this should be easily remedied by careful choice of where the Finish is placed.