I have been using the Wifi + Cellular iPad models since the first iPad Mini 2 was launched then moving on to the 9.7" iPad Air and then to the 12.9" iPad Pro.

Finally, I thought that I would like to just tether from my iPhone and bought the new 9.7" iPad WIFI model only. It was only through this new experience did I realise what I am completely missing.

1) Yes, tethering from the phone is one possibility, but it definitely drains my phone's battery a lot faster to tether
2) Because of that added step of trying to tether, I find the iPad less of a productivity tool and more for just media consumption
3) When I went overseas recently, I couldn't get a local data sim card and use it on my iPad (extremely useful tool to have)
4) Stopped using my iPad more of a note taker because it takes too long to connect and then open a new note

Just my 2 cents

yes, unlike one uses mifi to share data.
With a MIFI, that means I have another device to worry about as well as figuring out to maintain its battery charge and all

if connect from iPad to iPhone via bluetooth, it is not too bad and it can constantly be connected unlike personal hotspot via wifi
Yes, that is the best way and also means you have to take additional steps. Also you need to take out your phone from your pocket in order for it to get a better signal. Additional risk if it is in a public space

yup, iPad with wi-fi and cellular can act like a good hotspot device. alternatively, if one has a mifi, this can be resolved.
Then you have to carry 3 devices, which makes no absolute sense.

why it takes longer time to open? i tot note even without connection, u can still create a new note, and it will sync when it has Internet connectivity later. Dun really get the longer time part.
Because when taking notes, you also refer to previous notes and also want to grab items from the web to include into notes. Having that is what makes taking notes with an iPad better, otherwise just stick with a pen and paper

I just added a sub line for my iPad from my current Maxis One plan, RM48/month for 5Gb of data.

Yup excluding GST, but I get to have the 5Gb of shared data across so in total I have quite a fair bit of data between my phone and my iPad including my dad's line (which I am paying for his sub line as well).