Having engaged in combat situations with both... I totally disagree.
FA-Off PvPers may be able to aim fixed weapons better BUT there are manoeuvrability factors that without additional analogue hardware they will simply be unable to achieve with any degree of precision. Where general flight is concerned, KB/M is completely underwhelming and lacking.
Simply put - if you restrict yourself to KB/M you will be at a disadvantage... period... using a HOTAS setup also does not prevent you from using other controls on concert with it though.
Further more, not everything is about PvP either. However, just try and perform some of the more extreme evasive manoeuvres with just a KB/M and you will find them lacking.
I actually ran a HOTAS solution side by side with KB/M when I first got one and KB/M is comparatively limited in manoeuvrability factors. While FA/Off may offset the limits a bit, you may actually find you can pull off similar manoeuvres without using FA/Off using a HOTAS.
Where mouse and stick is concerned, I am not convinced... it would feel extremely unnatural for starters (most sticks and mice are intended for RH use). However, there are some sticks with a mouse "dimple" on the throttle - never liked those things myself.