They may use journals as a place to brainstorm and plan for more formal writing assignments, to set goals, and to respond to the teacher's commentary on returned essays. Journals promote writing in a myriad of ways both in and out of class. Sure, some of the writing will be weak, but this is to be expected and not condemned. We don't have to read everything they write. In fact, if we insist on reading everything they write, students will not be writing enough.
Journals provide a safe place where students can develop their voice and exercise their thinking.
Ultimately it is not about whether to use journals or not, but more importantly, to be able to articulate how our decision about journals fits into our teaching philosophy, pedagogy, and practice.