I have mine as for different uses. For the phone, I use it for:
Phone (Obviously)
Text Messaging/iMessage
Camera
Music
For the iPad, I use it for:
Surfing the net, when possible
Videos (TV Shows, Movies, YouTube, Netflix/Hulu)
iPad only games, like KOTOR, Eclipse, etc.
Social Media
Reading books & Writing
While all of these can be done on one device, I personally don't like it. I don't have my iMessage linked between devices. I can surf the internet on my phone all the time, but when I have the iPad with me, I rather use that. Videos, only kept on the iPad but I can watch netflix, youtube, etc. easily on the iPhone when I need to. Social media, I check it more often on the iPhone but I can still do stuff on the iPad but because of that, majority of my notifications is turned OFF on the iPad.
In all, it comes down to what you're used to. Everything I do on the iPad, except phone, can be done on the iPhone and vise versa but I separate them out.
The advantage of the Cellular iPad, I can surf the net using the iPad while my friend's driving instead of limiting to a small screen
