See, I had exactly tried this. I used my tablet for my Internet applications, gaming, etc, and started using an ordinary phone. Benefits have been clearly listed out by you.
But I faced the following problems:
1. It was difficult to carry my 10" tablet everywhere. So gradually I took my tablet to only very essential places and it in turn I could not be online whenever I required.
2. Dialling in dumb phone was not as easy as in smartphone, due to super smart dialling. It felt like going back to olden age.
3. Sending SMS was cumbersome in dumb phone .
4. The most important point to me was the call quality. The latest smartphones have superior voice clarity, which an ordinary phone can never offer.
The above points made me to switch over to smartphone again.