Although this is an relatively old thread, the topic is still very relevant.

To answer the question with various reasons:

1) Because this is America and those that want a manual safety can have one. Don't like the freedom to choose?...move to another country.

2) Manual safety is another layer of protection against ND. Those with kids may want all the safety features they can get.

3) Those that thinks the only needed safety is the one between their ears are delusional. No human being is capable of functioning at 100% mental capacity with awareness and judgement at all times. There are periods of lapse of judgement such as distractions, during waking up from sleep, etc.... In darkness, when one has to grab the firearm for whatever reason, a manual safety can potentially reduces the mishap of a ND.

4) New/inexperience gun owners...enough said.

I have both safety and non-safety and prefer the manual safety more and once it's holstered, safety goes off. The odds of discharging a firearm in self-defense is astronomically low, coupled with the fact that an accidental engagement/malfunction of the safety during an SD/HD, the odds of that happening are immeasurable.

I got a non-safety M&P9 Full Size because the ambidextrous safety is large and gaudy, more prone to unintended engagement/disengagement.

EDIT: Those that throw insults because of differences in gear or choices are ignorant and poor manner.