I see you do have quite the collection haha
I have the rMBP, 5s, AirPort Extreme and an Apple TV. I think I still have a 60GB video iPod and 4GB iPod Mini laying around somewhere.
It's just that I've never had an iPad for more than a few days, only iPhone's and recently a Mac. I'm trying to think of things I could do on the iPad that I can't do on my iPhone and Mac and I can't really think of any. Kinda hurts justifying spending $650 on one you know?
iPad I didn't buy, but was given as a gift and I really appreciate it more then anything else I have been given....
I travel a lot for work and I was in the habit of taking my MBP everywhere, and it is heavy (15"). iPhone of course goes with me wherever I go. But after I got given an iPad, I do not carry my MBP anymore. In my line of work I can store artworks and documents on my iPad, which is sufficient for my needs and presenting it to people I meet with. I can store movies, books to read and of course catch up on news. Then there is iMessage, FaceTime, Mail, Skype, etc etc for ease of communication.
On personal level, the iPad changed my browsing habits. I was using my iPhone for light browsing OR had to fire up my MBP for heavy browsing. But with iPad, i can do heavy browsing on my iPad and leave my MBP at work.
If you get an iPad, you will find you will start using that as your choice for browsing and reading stuff quite a lot. iPhone screen is too small for extended reading and browsing, specially if you are getting on of age (like I am) and eyesight is no longer as good as it used to be..
For me iPad is really the best addition to my collection of Apple's... so much so that I bought one for my wife and she too is well pleased. It's the only thing I have done right, since I have been married.... so she says....
I am well pleased.... Its the best $$$'s I NEVER spent....