Delivering constructive criticism in a polite and non-abrasive way is more efficient. If abruptly telling someone their wrong causes them to take offense to the help you're trying to provide, they'll likely disregard what you've just told them out of pride or spite and you'll reduce their willingness to work with you on future problems.

By being polite you're ensuring that the message you're sending gets incorporated into their strategy for tackling that problem and making them feel comfortable discussing other issues with you.

As programmers, we have to learn that 'programming' people is very different from programming computers.