why setting 'sig_on_top' is not recommended?

[Zhiqiang Ye <yezhiqiang@xxxxxxxxx>]

Hi, all

     In my opinion, when I reply email, the signature information
included before any quoted or forwarded message would be more clear. 
We can see clearly who says what, especially when we have a long
thread disscussing some topic.

     But when I man muttrc, they say:
 sig_on_top
              Type: boolean
              Default: no

              If  set,  the  signature  will be included before any
quoted or forwarded
              text.  It is strongly recommended that  you  do  not 
set  this  variable
              unless  you really know what you are doing, and are
prepared to take some
              heat from netiquette guardians.

             I don't why this is not recommended. Can anyone help me out?
             Thank you very much.

Zhiqiang Ye