I think what lyonaria is saying is that anything surplus to the requirements is unnecessary and can just cause confusion if your case is straight forward. I think some people think "the more paperwork, the better" (and I used to be one of those people!) but in reality it really isn't necessary. A letter would be necessary to explain anything that needs clarification. If it is straight forward, then a letter is not needed. I know it sounds bad, but I don't really think they care to know more about the person behind the paperwork. The marriage certificate, financial information and accommodation information will say all they need to know, especially if this is your new FLR(M), so you've already been through it before.
If you have an in-person appointment then you definitely do not need the letter but yes, your spouse should be there, because if anything arises then you are both there to discuss it. You wouldn't want to take the chance of having one little thing that may be misunderstood and you didn't have your spouse there with you to help answer any questions.