"Effectively to use this feature, you will block 7–9 of the 13 channels and, of course, ch12 and ch13 aren't available in North America. So yes, it's possible that your neighbors will hate you if you turn this option on. This is why there was a lot of pushback at the end of the final IEEE 802.11n ratification to entirely ban 40Mhz in 2.4GHz. That didn't happen as 40MHz is still appropriate for some situations (ie: a warehouse) but the bottom line remains, enabling 40MHz in the 2.4GHz band isn't a good idea for dense residential areas."
40MHz only causes problems for me in my area, and I live in a neighborhood. I can't imagine the 2.4GHz saturation in an environment like apartments. If I ever get a bunch of land and a house set way back on that land, I may enable the feature, but as it stands, I don't need to take up the extra channel, nor do I desire to cause additional interference in my area.