So many times, my clients want witnesses to just be able to write a letter rather than appear in court. Unfortunately, this is not a college application, and your witnesses need to be in court. You aren't asking for a reference, you're asking for someone to come to be a witness in your case.
Anyone can sound articulate in a letter which has been spell-checked and edited. The "witness" may not have written the letter, and may even have signed something written by a party himself or herself.
A judge wants to see your witnesses' demeanor. He or she wants to see whether the person can answer a question directly or will wander around the topic without truly answering. He or she wants to see how truthful your witness is.
Opposing counsel wants to be able to test the statements made in the letter. Opposing counsel has the right to do so. The hearsay doctrine makes letters not admissible. Lawyers in a case might agree to allow in some hearsay letters, but most want the right to ask follow-up questions of any witness.
The judge and opposing counsel are going to want to know how much this witness has had the opportunity to observe. I have had several custody cases lately where the father has brought neighbors in to testify about his parenting. On cross-examination, I discover the neighbor has never been in the father's home and has had truly limited opportunity to observe the opposing parent with the children. Had the information been submitted by letter, it could have seemed more impressive. On cross-examination, however, I can make the point that the neighbor really has very little insight into the father's parenting.
It is difficult to coordinate witnesses for a trial, and you are often asking them to take time from their work and families. Don't be shy. It is crucial for your case.