I think there is an important point that many people who argue against being against smartphones are missing.
It is something that they may not be able to understand, and there is a good reason why it may be hard for them to understand.
The issue is that they think that the only problem with smartphones is that someone may log onto a very inappropriate site. What they don’t realize is that there are many other problems with smartphones besides that.
Some of these have been already been mentioned above, but I would like to mention another one:
As Torah Jews, we should strive to live lives that are “kulo Torah and Kedusha”. When someone in online, they are exposing themselves to something other than Torah and Kedusha. I am not talking about “really inappropriate sites”. I am talking about just about any or maybe evem any site.
No matter what you are doing on the internet, there is a very good chance that you will see an ad or aheadline or something that contains something untznius or non-Torahdik. I don’t know if I have ever logged onto my computer without seeing something that was in someway non-Torahdik.
Regarding the argument about not walking on the street for the same reason – guess what? I do try very hard to avoid walking down streets on which I may encounter something non-Torahdik, and I do try to avoid public transportation or stores that may involve the same.
The people who argue against being against smartphones are probably people who are not trying to live a life that is “kulo Torah” and probably do not try to avoid such things in general (in terms of streets, stores, and transportation,etc), so that is why they have trouble understanding this.
Everyone is on a different level when it comes to these things, but as Torah Jews, our goal should be as much as possible to strive to live a life that is “kulo Torah” in every aspect. Just as we try to avoid streets and stores that may involve inappropriate sights and sounds, so too we should try to avoid smartphone and internet.
And even if one is not holding by it, they should understand that it is the goal, and appreciate and respect those who are holding by living such a lifestyle.