I login into sites with just a tap on the iPhone. With other iOS password apps you can copy and paste either the url, username, or password, but you can't do all three with just a tap, with 1Password Pro you can.
I use both desktop and iOS apps.
Desktop makes saving new urls, usernames, and passwords extremely easy.
iOS app simply makes filling out all that stuff on sites as easy as a tap.
You'll need the free version to get the DropBox sync going though. If you don't need the desktop, it's not necessary to keep it. I'd still go with 1Password Pro even if I hadn't bought the desktop only worrying that the syncing code might be changed (syncing code for other services like Gcal and Toodledo occasionally are changed, which then creates problems for apps that sync to those services).