Great question ... Frankly I am not a huge iPhone fan but it's necessary if I'm out and not carrying my iPad ... However otherwise my iPad is always nearby. .. But I love love love To be able to pair it with either the iPhone or iPad cellular.
I think you're wrong. Everyone I know is always connected to wifi or LTE or both. Didn't Apple pushed the concept of seamless interconnection between all their devices i.e; handoff, notifications, etc;
I own iMac, MacBook, iPhone, iPad and all always talk to each other, Apple watch will not...
In my opinion Apple specifically designed the Watch to work in conjunction with the iPhone just like smart watches by other brands work with phones of the same brand, the Pebble of course being the exception and working with both Android and iOS.
There is no chance they will make it compatible with iPads because not all iPads have cellular connectivity. Those that do can't make phone calls and typically are on pay as you go with limited data capacity. Personally I don't see the point in iPad connectivity and even if it existed would not use it over the phone connectivity. I don't always have my iPad with me and the same applies to everyone else I know most of whom only own the Wi-Fi only model.