My lawyer also told me not to send AC21 proactively as the letter never attaches to your file. He told me to wait if USCIS asks for employment letter, then send AC21 letter along with your response to RFE. Thats exactly what he did in my case.
The adjudicator will send RFE to you with any document he thinks needed for your approval. Whenever an RFE is issued, that is an indication that your file in under review que and with an officer. 99% of RFE demand for letter from "current employer". USCIS adjudicators are Lawyers and very careful with words and the applied law. They know everythng about AC21 (I140 approved +. USCIS can not invite lawsuits because of incorrect application of AC21 and denying a case.
I agree with GINNU for Pro-active approach, but again, there is no concrete benifit.
In the event you get RFE, you will eventually send employment letter and AC21 cover letter. Most commonly, people who sent the letter pro-actively did not help them from getting RFE. There is no check mark in USCIS system for AC21 application that is why there is no LUD even when you get a return receipt of AC21 letter.
Yes ... in the begining of AC21 saga, when the law was in much more confusing state, there were af ew instances when AC21 cases were denied, but they too were re-instated after MTR.
Being pro-active is good when results are sure.