The point of the Reset Master Categories ID is for those who import their data and do not set up master_categories_id ...
Or, for people who converted from other software that did not use master_categories_id ...
Or, for people who some how managed to destroy or other damage their master_categories_id ...
This method will fix the master_categories_id 100% when there are not Linked Products ...
If there are Linked Products, it can probably fix them to 50-90% depending on how things are set up ... then, the other categories can be fixed in the Multiple Link Categories Manager ... where you can pick individual Categories and reset all Products in that Category to use it as the master_categories_id ...
So say you import your Products with easy populate ...
First, you probably have no master_categories_id ...
So, you run the Rest Master Categories ID ...
But, you know that your 12 Designer Categories should be the master_categories_id ... so you go to the Multiple Link Categories Manager and reset those 12 Designers Categories to have the categories_id used as the master_categories_id ...
The bulk of the work is now done ...
If you find other Categories that have an issue, you add those to the list that need to be reset ...
Once done, you then run the Update Products Price Sorter ...
The point here is instead of editing 10,000 products you were able to fix this by utilizing these tools to get the bulk of the work done for you ...
Perhaps, once you fixed all of the master_categories_id and all were right ... you could figure out how to fix the Export/Import of Easy Populate so that they did not break in the future ...