It is a "strongly recommended, not a mandate. Other than the obvious benefits there are other things to consider.

1) The red aiecraft use only flag clearly defines the end of your antenna, avoiding being stepped on, or poked in your eye.
2) The red aircraft use only flag makes an unextended antenna more visable so someone can warn you that you have neglected to pull it up.
3) Frequency markers enhance frequency control. Others will know the channel you are on and less likely to shoot you down.
4) Frequency markers, particularly personalized ones, identifies your transmitter from the numerous others that look just like it.
5) Frequency markers with your name on them increases the licklyhood of your transmitter being returned should you forget and leave it at the field.