this problem is haunted me long time: when subdividing just a selected block then close block is affected too with nonsense slices, BUT how to subdivide without slices?
on picture there is red marked slices that i do not want when subdividing but selected area is ok like i want. This is very nasty thing sometimes. I do not know Blender fully so maybe i have missed option to subdivide without side effects.
Currently blender only permits faces to be made of 3 or 4 sides. Those extra edges are needed to permit this otherwise you’d have a five sided face created. You can look at using a bmesh build from graphicall.org but still pretty buggy
You can select adjacent tris and hit F key… it’ll convert to a quad. You’ll still be left 2 tris in your picture.
instead of subdividing ( W > Subdivide) make edge loop cut (ctrl+R). Blender will make in all into nice quads.
Would this be too much of a hack?
Select a face, extrude (E) -> Right mouse button (RMB) click.
Subdivide it.
Select all faces (A), click Remove Doubles from the toolbar.
heh… only looks so nice… look what happens when manipulating the mesh…
Well yes, some “hidden” and potentially troubling faces still remain in the mesh, that’s why I asked if it was too much of a hack 
Still, it might have some applications even though it’s only “faking” ngons.
good one anyway:D may be useful:D
thanks all for info, bmesh made it like i wanted but currently i just use hacks because bmesh is not ready and is not included in official Blender.