When not to use 'track by $index' in an AngularJS ng-repeat?

I recently got the console error `

Error: [ngRepeat:dupes] duplicates in a repeater are not allowed. Use 'track by' expression to specify unique keys...

— AngularJS Error Reference - ngRepeat:dupes

which I then used 'track by $index' and the issue was solved...

But that got me thinking... is there a reason why you wouldn't want to use track by $index in an ng-repeat?

I've read SO questions like this one as well as other articles, but it seems like almost all of the articles only talk about the advantages of using 'track by'.

Can someone list the disadvantages and give an example of when you wouldn't want to use track by $index?