Five Sentences? Really?

If an e-mail can reasonably be answered in just one straight-forward sentence, so be it. But if an e-mail really requires a longer response why is it a bad habit to respond with a long answer as long as you manage to make your point and settle the issue? I think it can be outright disrespectful to put an arbitrary cap on the number of sentences in an answer to an e-mail.

Q: Why is this blog article five sentences or less?
A: http://five.sentenc.es