It would force the same behaviour as the batch file does, so that the batch file can be omitted and maybe used for other purposes.
And by removing the batch file, the short flickering of the console windows would disappear as well.
"AutomaticMerge" would be set to "true" by default, resulting in the "normal" behaviour in a merge operation. But if it was set to false, it would block the automatic merge and force a conflict, just as the batch file does.
If you decided to implement that and find, that is an important option, you even might put it into the "dialog options" of TSVN instead of the "advanced options". And if it was even more important, then you might directly add it as an option in the merge dialog, so that you can choose the desired behavior on every merge.