B737900er - I don't know what the latest FMC update does, but Boeing always used to recognise '6000ft' as a 'useable airport in terms of what is displayed on the map page (737). I think even for the 900, that is fine - as de f says, your actual normal landing distance is small. Any deviation from 'normal' in terms of a/c or weather, then into the QRP. There could well be occasions when 8000ft is not enough!