I thought to pose the question to other artists, artists schooled in traditional methods and materials, and to inquire of them as to whether they would think to use a tablet computer for sketching purposes. Surprisingly, none said that they would, pointing out that they were much more comfortable employing methods and materials found reliable over decades of use out in the field.
As for myself I'm still debating whether to include a tablet computer as one of my sketching tools. For the time being, however, I think that I'll carry on with pencil and watercolour. There's just something about the way that pencil and watercolour react with various types of paper producing a feel and effects that , despite the wonders of technology, a stylus and a tablet computer have yet to master.
Oh, and as for my question about whether digital creations are art, of course they are. Art is art regardless of the tools used in its creation.
Algonquin Landscape Graphite Drawing
Algonquin Landscape Watercolour
Algonquin Landscape Graphite Sketch II
View From Lookout Trail Algonquin