Q - Do you believe every writer should have an agent?
A - The short answer is No. But I'll give you a longer one. First, you must be writing books for an agent to be interested in you at all. Agents don't typically handle articles, short, stories, or poetry--except occasionally for their current book clients. Although the market guide indicates which agents are open to submissions from new clients or unpublished authors, they will expect you to have a completed book manuscript before you approach them.
Generally speaking, you will have a better chance of finding an interested agent if you have a book with a fairly broad possible reading audience. If yours is a niche book--with a very specific audience--you may do just as well finding a publisher on your own. Keep in mind that, from a financial standpoint, an agent will be looking for projects that will make enough money that their 15% is enough to make representing you worthwhile. That's just the reality of the business.
At this point, it is only the major houseS that require an agent, so with 450 Christian book publishers listed in the guide, you certainly have a lot of publishers you can approach on your own.