I have been writing in journals with pencil for many years now. In fact one of my more popular and circulated posts was correcting the false notion that pencil fades over time. It doesn't. For a long time I used mechanical pencils and was quite happy until Steve McCoy sat me down and introduced me to the world of wooden pencils. So this is all his fault, really.
My preference for a pencil is not the ability to erase (though that is pretty great) but the act of writing itself. Laying down lead (I know it's graphite and clay) on paper is a completely different experience than writing with pens. On a very simple and subjective level, pencil feels better to me.
When Steve gave me a pile of good wooden pencils last year everything I loved about writing in pencil was amplified. Varying hardness and blackness of leads allows me to change things up for doing different work in my journals. Bonus: the smell of the cedar. And of course there is the hobby aspect to it. I don't have time for a real hobby, but pencils are an inexpensive interest that make writing more fun.
So, yeah, I have a pencil roll in my bag these days and a bunch of you have asked, "What's up with the pencils?" I just like them, and find myself more creative when writing with them.