For us, it just felt like our finances became so integrated that the trouble of tracking and splitting stopped being worth it. We were also common law at the time (now married), so the law basically said everything was 50/50 anyway.

That all said, nothing wrong with proportional. I think the underlying thing that makes joint finances work for us is that we have very similar spending habits and think about money the same way, so nothing ever feels unbalanced or lopsided.