hi , the 2.4 gig wifi world is saturated with users and so as to ensure that this doesnt continue to happen the wifi alliance set a rule that forces the 2.4 gig down to 20mhz in areas where the 2.4 gig is congested and to avoid further congestion , its also called neighbor friendly wifi
so if you disable this setting and force it to run at 40mhz you are just being rude to your neighbors and pretty much giving them the bird
so you can be nice and leave it enabled and everyone will get a bit of bandwidth without too much congestion or you can be a prick and disable it and say screw you to your neighbors
thats your choice
your older 2.4 gig devices will work regardless of if its running at 20mhz or 40mhz
here is a link to help you understand why its there and what it does
