Usefullness of Study Guide
Goal-oriented action
Part of this really depends on your goals in administering the exam.  If your goal is the traditional idea of creating a nice bell-curve to rank students, having a study guide makes very little sense. You can separate the wheat from the chaff without providing any assistance to the students prior to the test.
However, if you want to utilize the tremendous cognitive benefits of testing to actively help your students learn, then you're looking at a whole different set of priorities.  There is a lot of research on the benefits of testing in the learning process.  Unfortunately, I don't have access to my bookshelf right now, but the incredible book How People Learn summarizes the results very nicely.1
How do tests help?
Basically, tests accomplish two things that really help us to learn.  First, there is retrieval practice, which is simply giving the brain a chance to practice holding on to (and recalling) information.  The second is emotional: by giving the information heightened emotional importance, we increase our chances of actually encoding it.
So, where does a study guide fit in?  Having a list of topics isn't as useful as you might think for an examination. If I tell you that you're going to have an examination on &quot;Operating Systems&quot;, there's not a lot to go on.  You might get to the exam and find questions about threading, or disk management, or you might find questions about how to transfer photographs between an iPhone and a Linux box.  There's little to focus your studies, and you may find that you have expended a great deal of effort studying things that your teacher does not consider to be the core, central ideas of the course.
My example is over-the-top, but this is done purposefully to illustrate the problem.  Your subtopics may not seem as broad as &quot;Operating Systems&quot;, but in a field of effectively infinite knowledge, you are presuming that students have gleaned what the important core ideas are for your class from readings and from listening to you during your lectures.  For some kids this will work.
But what about the kid who missed something?  Perhaps they didn't understand something really fundamental about one of the sub-topics.  Or perhaps they got sick and missed a class.  Or perhaps they've just forgotten some things thoroughly enough that they really can't recall what the important ideas were.
Creating a good guide
A study guide is your chance to give some very gentle guidance.  Take your syllabus, and write a sentence or two for each topic that starts with, &quot;Make sure you understand ...&quot; or &quot;You should be able to ...&quot; or &quot;The most important idea here is ...&quot;.
This is a relatively easy task, but even this small amount of guidance will help your students to focus their studies on the most important ideas of your course.
You can actually go a lot further with this effort if you want to, and there are good reasons to consider putting a lot of effort into the guide.  Your efforts here can have an outsized impact on your students.  This is what they will study, so what you put here is what your students are likely to remember when they leave your classroom for good.

1 - All I could find online were a few articles about single studies here and there.  This is probably the best I can do for now.