Why must invoices have > $0.00 value?

[Marc Evans]

Hello,

I am trying to understand what the limitation is in the system that 
makes it necessary to require that invoices have a > $0.00 value? I ask 
mainly because I would like to use negative values for debit memos. As I 
look through the system I am not seeing an obvious reason for the 
limitation, and am considering removing it. Can anyone speak to this?

- Marc