This scenario does not actually play out in practice, because in order vote tactically in this way you need to be able to predict in which order the candidates

This scenario does not actually play out in practice, because in order vote tactically in this way you need to be able to predict in which order the candidates will finish. If you're wrong - say, by too many people voting in the same way as you - you can could easily find your the candidate you ranked #2 defeating the candidate you ranked #3 (that you really preferred) with the help of your vote. So yes, there are times when after the election you can see that you could have voted tactically for someone other than your most preferred candidate in order to assist them, but you can't easily or perfectly predict this beforehand.