My first sketchbook was done mostly in pencil. Pencil seems to come to mind when the word "sketch" is concerned and, to most people, the word "sketch" also mean the type of drawing for draft or temporally use. Pencil is probably drawn into association with sketchbook for its non-permanent marking property that correction or elimination are possible when required. But for sketchbook, the recorded entries can be either final work in itself or a quick take for future reference, depends on personal intention, therefore, such flexibility is no longer crucial. On the contrary, the more lasting quality of the ink might be more desirable for recording purpose. It is, indeed up to personal purpose and preference of the individual whether pencil is used for the sketchbook.
Pencil produces flexible, sensitive lines as well as broad range of shades.
Lines created by pencil certainly have unique charactoristic not possible with ink and pen. A line created by pencil can be sensitive and varied in tone and weight. Pencil can also be used to create a plane or block of tones and I use pencil for this quality more than just for line work. I use only soft pencil of 2B onwards and with its lead shaved into a square chisel shape, like the carpenter's pencil. By the way, I do not use carpenter pencils at all since they are not soft enough.
Sample of pencil sketch.
Over all view of river houses in pencil sketch.
Detail of the above showing broad pencil stroke using chisel shape pencil lead.
My pencil sketches would make up of some pencil lines and mostly shapes created by tone and shade using the flat sides of the chisel shape lead. I also use lots of blank spaces provided by the white of paper surface to help compose the subject of my sketch. Lines are used in between these tone shades of pencil and the whites of paper to define a desired shape and for emphasis where and when required. It should be noted that while the chisel shape lead technique is very appropriate for sketches of architectures and building in landscape, it is also versatile enough for most other shape, subject and situation as well.
Pencil used to suggest intricate details.
Once again, it is most appropriate to say that the choice of pencil or pen for sketchbook is entirely up to personal preference. I personally find that pencil sketches tend to require more time in building up of tones and shape while ink line takes less time to realise the outline of the subject but lack the mass and depth of pencil sketches. However, if there's the intention of adding colors to the sketches, then, you would find that ink work is more flexible and work well with both watercolor and dry medium where pencil sketch could be more difficult to work in watercolor washes.