Re: your final 'yes', um, no. Just because one can tether doesn't mean it's an optimal method of connecting.
I travel a lot. Yes, tethering ears phone battery. But the biggest plus in my opinion is the ease of use. I don't have to tether, don't have to set up a hotspot on my iPad. All I have to do is press the Home button and go. I don't need to dig my phone out of my purse, don't have to make sure it's within a certain distance for BT to work, don't have to worry about the connection dropping, etc.
Some people may decide that their usage is low enough that tethering is the best option. But there will always be people who have reasons not to go that route. Personally I think eventually wifi-only models will go away and all tablets will be cellular-capable. Will Apple do it? Don't know because it's hits their margins. But I do believe they will eventually get there. If for no other reason than to streamline their production/ordering process. I guarentee they have already run the numbers.