The Star Wars books arent a great source. If the Imperial Star Destroyers are really that powerful, they wouldn't need a Death Star to blow up planets. just a dozen destroyers would do to reduce the planet surface to rubble.

Teleporters have limited mass capacity. Otherwise, Starships would not need shuttles. Backpack nukes may be possible,
The Death Star is not only a WMD. It serves as a symbol of the Empire's might and as a deterrent to any systems that may rebel against the Empire. As Moff Tarkin said in the movie... "Fear will keep the local systems in line, fear of this battlestation"

Even if the Empire never even showed themselves in a rebelling world, the thought of the DS suddenly coming in-system and blasting your world would be enough for the local system government from rebelling againts the Empire.

Also blowing up planets is extremely counterproductive as you loose the planet's natural resources, manufacturing/industrial bases, manpower etc. Alderaan was blown up as an example of what will happen if you cross the Emperor.

Regarding teleporters in Trek, they are also very unreliable and unpredictable with results like turning Picard and the officers into kids in TNG or merging 2 people into one like Tuvok and Neelix in VOY.