While lot of people say it is needed, I got mine without one. Primarily, they are looking for Employer Employee relationship. While I applied for my H1-B, I did get a RFE to establish Employer Employee Relationship since I did not have a client letter. However, my client was adamant not to issue one because of their policy. I responded the RFE with Purchase Order, email from few colleagues confirming that I am a contractor working on a project. I also made my manger send me an email denying Client letter stating :

legal informed us that the letter you requested cannot be completed/provided since you are a consultant working on a XX project and not an employee of (Client Name).
As (Client Name) is not your employer and doesn’t manage you, your pay , tax, benefits etc; you will need ( Current Employer) to complete the requested letter since they are responsible for your employment & the letter.

I also had my vendor write a letter that my current employer and they are in agreement to provide me as recourse to that project.

Hope it helps.