It's also a robust PDA, MP3 player, syncs with my Kindle, a [weak] game system, a decent enough camera for quick and dirty snapshots and a great source of information on the go. I also use it for dozens of other things you can't do with a dumbphone which may or may not apply to you (scientific calculator, lookup tables, brewing calculator, recipe management, backup USB stick, etc). It has supplanted my need to carry a laptop in many situations too, and when it hasn't, it has provided tethered internet access.
It may not outperform dedicated devices for each task, but to carry all of those devices you'd need a backpack of gadgetry with you at all times. The extra cost per month for me is easily outweighed by the convenience of having all those options in my pocket at all times, and cancelling my home phone basically negated the extra cost (although cost isn't really a factor, it's about the same as a decent meal in a restaurant).