when importing data there is only one merge option: it will merge terms and fields combining them all together. In some cases you import terms with specific descriptive fields referring one to the other (source to status etc.). So after import you cannot see which group of descriptive fields belongs together. I know that you can manage nested fields (text field depending on a picklist field or vice versa) to create a clearer hierarchical structure but this makes layout and input models much too complicated.
Therefore in some cases it would be useful to merge entries while keeping term groups separated (e.g. by adding numbers to identical terms).