I own a MBP, iPad mini, and iPhone 5s - and use them all with a good amount of overlap in functionality, but all in different scenarios.
Most of my time is spent on my MBP. I do things like schoolwork, programming, 1hr+ web surfing, photo management / editing, and chatting.
I use my iPad for a lot of media consumption. I surf the internet, read articles, magazines, and books, watch YouTube videos, Netflix, and other video apps, and play games. I use it most often while laying in bed before I fall asleep or on the couch while watching TV. I also take it with me whenever I'm doing something that will require a 15+ min wait time, like traveling or appointments (DMV, doctor, etc)...
I use my iPhone for phone tasks (obviously talking, texting), taking photos, social media, listening to music, and very very light web surfing. I like to use the iPad to do the web surfing if I'm going to be on it for more than a few minutes.
I don't know how to answer the "worth it" part for you. I love all of my devices, and to me, they each feel right in a ton of different scenarios. Sometimes it feels redundant to have all 3, but I use them all extensively (daily), and don't regret it at all.
EDITED TO ADD: I even own a desktop (Mac mini) - that's the only one that feels a bit redundant (on the border of getting rid of it), but even then, it has a specific use. My Mac mini rarely gets touched for daily use, but I use it as a media hub. I only sit at the desk to use it when I'm going through my iTunes library, pretty much. For all other computer related tasks, I just use my MBP.