I’m quite fond of representative democracy, and don’t think replicating the Swiss or Californian system would be a particularly good idea. I do however think that referendums are an occasionally vital and necessary part of democracy, and to do away with them, like the German constitution does, would be a great mistake.
There are situations where referendums are the only acceptable alternative. As a supporter of representative democracy I disagree with people who say that this or that issue is too important to be dealt with by the normal electoral process. But I do think I think referendums are necessary when an issue is 1) divisive 2) vitally important and 3) the normal partisan system cannot properly deal with, because the fault lines are different. As a corollary, anytime sovereignty is involved, I think an issue has to be pretty minor for you not to hold a referendum.
Most of the referendums on EU memberships are textbook cases of this situation. In the case of Sweden, nearly half of voters opposed Swedish entry and for most of the campaign the no side led. Without a referendum they would have had to vote for the Green or Left parties if they wanted to stop our entry. Both quite radical non-mainstream parties who together held less than 10% of the vote. In some countries all parties were for membership. In these instances I feel not holding a referendum would be undemocratic, and would to some degree disenfranchise (to use an American term) the whole electorate.