*Increase your phone battery life because you'll likely do all of your "smartphone things" on the tablet, reserving battery for phone calls. (doesn't apply to light users)
*Do research/get info faster with a larger screen, less zooming and panning around.
*More enjoyable movie watching while travelling
*Easier to see GPS while driving
*Working with office documents is easier (excel totally)
*This one depends on your ability/device but the larger keyboard is easier for me to type on compared to my phone. I make more mistakes with the smaller keys, then again I hate virtual keyboards in general.
Basically most things center around the larger screen yet it's still very portable (it's in the front pocket of my khakis right now while I work). Think of it like this, what things do you do on your Droid X that will be more enjoyable/faster to accomplish on a larger screen? With a 7" tablet its the reverse now. I think "what can my smartphone do that my tablet can't?" Okay make calls, I know.

Ultimately, only you can decide if a tablet will improve your "smartphone experience".