I asked two close friends to do an affidavit supporting our I-751. Their immigration attorney wrote them an e-mail. In it he advises then not to do it and implies there is something strange about our request because my spouse and I obviously are not willing to rely on "primary evidence." We can show joint residence, joint bank account, joint utility bills, etc., and we will. But don't we have to also file an affidavit from two people who have known us and know we are a married couple?
There does not seem to be anything strange about including Affidavits of Support in your I-751 petition to remove conditions on the green card. I submit them as part of my routine practice. The Affidavits are often included along with primary supporting documents but are not absolutely required. Specifically, the instructions say "The documents should cover, but not limited to, the following examples" and goes on to list the evidence in support of the relationship to submit along with the I-751. If USCIS wants to see more evidence they will issue a Request for Evidence.
However, along with your evidence, you should also include current family photographs taken since the green card was issued and through out your relationship.
I do not know if these friends of yours are US citizens or permanent residents or if they have another immigration status. It is best if the affidavits are from US citizens or permanent residents, Your friends' immigration attorney may not want their names on a document where they could be called to testify to its accuracy in front of an immigration officer.
I hope this answers your question.