If you get the Smart Keyboard for the 10.5” iPad Pro, that makes its extremely slim. It combines a stand, keyboard, and cover to make a thin profiled device.
My current setup is an iPad Air 2 with an Apple Bluetooth keyboard. I have a keyboard case to protect it. All of this fits inside my Tumi messenger bag. My MBP 13” wouldn’t fit in it.
I am thinking of getting an iPad Pro 10.5” with a Smart Keyboard eventually but the iPad Air 2 is still functional for me. So I can’t upgrade just yet. The only real incentive for me is if I am going to start doing more work editing with a photo app or graphic work with a drawing app. It’s hard to describe it but it just feels different using your finger or Apple Pencil directly on the iPad screen instead of a mouse or trackpad for drawing. I feel like I’m actually working on paper instead of trying to match my finger on the trackpad to the cursor on the screen.
I love the iOS to Mac handoff feature to be able to work on the same document on my Mac or my iPad.
I also use Moon Invoice app on both my Mac mini and my iPad. I thought it would be interesting to use the Apple Pencil for customers signing invoices when I’m outside the office. But it feels like a luxury right now.
My daughters already have ipad minis for themselves. It was a simple matter to get them their own Bluetooth keyboard and they can use it for their school projects instead of buying a MacBook Air. They just have one device instead of getting an iPad and a MacBook Air.
If you’re happy with the MBP 13”, stick with it. Ugh you need to find a role for it. You might not need it now. Revisit this idea in the future. Or just get an iPad and tinker with it. You won’t know when to use it until it finally bonks you right between the eyes.