4-speed manual?

I know, i know. If I wanted a manual, I should have bought one....but I didn't. The manual says that manually shifting between 1, 2, D3, and D4 you can drive your vehicle much like a manual without a clutch. I've done this on many occasions and I like the way that the car feels when I have control of the gears. Is this dangerous to the engine or tranny in any way? My automatic transmission will redline 1st-3rd gear by itself anyway.
Besides for having an extra gear, is a manual still better than driving my accord like a 4-speed manual?
Also, since I've never driven a manual, I have no idea what "double clutching" is. I've heard people mention it, but I don't know what it does for a manual transmission vehicle.