Yes I do understand, but there are some situations where it might work out.
For example when a team CT's with a high enthusiasm they still aren't chanceless for the next round, or when a team knows you are the biggest team to defeat, and expects a easiers team the next round. You can CT in the cup and then TIE a few times in the league (if you want to get the cup and don't care that much about the few league games that week), then the CT in the cup could be really effective.
This works every round except in the week where there are 2 cup matches.
I agree with you on the playoff part though.