Sit-ups don't work a balance and to a large degree work the hip flexors in a short range of motion, including the iliopsoas. The iliopsoas, when short and tight is a main cause of postural imbalance and often a cause of back pain.

According to Parore its the main muscle in your body you want to keep loose and healthy. Makes a big difference in sex too :)

With Western life style of always sitting, which causes the hip flexors to shorten, combined with training it short by old school ab training (running also shortens it) its certainly a consideration to think about I think. Never mind that I believe there are more effective ways to train the abs/core etc even the 6 pak itself.

Sit-ups affect breathing to. As an athlete breath is extra important.