Fragmentation has nothing to do with scans (other than slowing them down if they're from disk), so I don't know what you're trying to say here.

Analysing whether or not an index is useless does not involve looking at the table size, unless the table is under a page (8k) in size. 12000 rows is well over that. Analysing whether or not an index is useful involves finding the queries which use that index, testing them with the index and without and seeing whether the performance change is acceptable.