I don't care for the photo uploading features, and I wish Dropbox (and Google+) didn't ask to enable it every few updates.
Dropbox works well on all three OS I use of which Linux is my main. I don't know what iCloud and Photostream (no syncing of videos?) are like now but it didn't really work well when it was introduced and I'm taking a guess it doesn't work on Linux natively.
I guess I'm the reverse of you: I've already got photo and video syncing with Dropbox so why use Photostream? If that feature wasn't in Dropbox's client, I'd rarely use the client.
But, I'll accept your premise - for the casual user of Dropbox, who doesn't ever want to buy apps for their iPhone (unusual, the vast majority of the value of an iPhone comes from it's ecosystem, but i'll run with it) - they can access their files through the iOS client.
But, really, everyone who does this seriously should have Goodreader on their iPhone.
I access dropbox from my iPhone and iPad constantly as well, but I do so from GoodReader, where I download files, organize them, read them, password secure them, etc...
I've tried out the new "Photo's" feature - but I already have 100% intelligent photo syncing between my systems with Photostream - so I stopped the camera from syncing after about 5 minutes - my photos are already in my Camera Roll + PhotoStream, and therefore in my Aperture Library. I couldn't understand how, or why, I would want to use Dropbox to sync them again.
This is one of those extraordinarily rare times where I think Apple's Photostream, with shared photostreams, and Albums, both public and shared - actually provide the "Web/Internet" services that I want.
Anybody else find a use for the iOS client for Dropbox?