The answer depends on your requirement, or replacement that you want to. Each has its own use. To minimize your compilation you have to use iPhone only. One of my friend use iPhone,but don't need iPad for other purposes. This a choice you can make, what you needs more. iPad is better for reading, watching videos, do some work, its best if you can allow the both
As far as having a dumb phone & an iPad. I had an iPad 3 and a dumb phone for a year. I used the iPad 3 for text & internet while using the dumb phone for obviously phone calling. It was cool and all, but then having the iPhone & iPad compliments each other very well.
I use my iPad Air as a laptop replacement. I have the Belkin Qode bluetooth keyboard case and it turns my iPad into a very productive machine. I also have an iPad Mini Retina that I use for a lot of multimedia.
This year I just bought an iPad mini, and I also have an iPhone 5s and MacBook Pro. I mainly bought the mini to take with me on a 9 month trip to Asia. It's good for watching some movies and reading books. Since I'm using the mini I got used to that bigger screen. It's so weird working with my iPhone now. I am seriously thinking of selling my iPhone now and buying the most simple phone on the market.
I carry a iPhone 5, an iPad Air, and a Mac Pro laptop. I travel a lot and carry all three with me. The iPhone is always in my pocket, the iPad and Mac Pro in my backpack. Each serve important functions. My iPhone is for calls and when I don't have my backpack. My iPad is when I don't have access to power and need to do serious work, like on an airplane or in a restaurant. My Mac Pro is when I have power and table to work.