Well, to be fair, cup games aren't more important than that for TUS in general (or vice versa).
But, if the opportunity presents itself to play your cup, you should still favour it over clan games because clan/single games can be played against anyone you like.
Cup games have to be played against specific people (which are not always easy to find)
Also, the league has a deadline of 2 months, while cup deadlines are a lot shorter.