quota-status fails when size=0 from Postfix

[dovecot at ptld.com]

> Plus Dovecot complains that the policy service is only supposed to be
> used in the RCPT stage. So clearly this is a bad approach.

I want to explore this more. I tried it and also see:

dovecot[1096]: quota-status(26164): Warning: Received policy query from 
MTA in unexpected state END-OF-MESSAGE (service can only be used for 
recipient restrictions)

Why? Why does dovecot even care? Quota plugin is sent a user and a size, 
it looks up quota for that user and computes if size will put the user 
over limit and returns an answer. Why does dovecot care or even know at 
what stage this is done? Why is it bad to check quota after getting the 
real size? Seems like its designed to allow spoofing from an evil mail 
client.

What is the harm being done that causes this log warning? What is the 
harm in ignoring the warning?