One thing is true: whatever your phone is, the more complex, the more features, the more risks there are. I really don't see the advantages of having a handsized computer, really. The performance/battery/usability/cost compromises are not really making it worth it. Most people do a lot of text messaging, usual smartphones are not designed for it. Old school, classic cellphone do it pretty well. Why would you need the…
For an example from my own experience, I used to do a lot of contract IT work, both freelance and as part of a firm, often visiting several clients over the course of a day. Originally I'd keep track of everything in an old-fashioned spiral-bound pocket notebook, but dead trees are hard to grep, especially remotely, and at the rate at which I filled notebooks it got to be a real pain having to copy and distill everything worth keeping track of from the old one to the new one. I had an old-fashioned candybar phone, too, which worked well enough for phone calls and text messages, and if I had to, I could use it to talk a client through power-cycling a server and other similarly simple procedures, which was at least a little better than having to go out and do it myself, or back to the office where I could remote into the client's PDU.
So I bought an iPhone. Thus, with a modicum of effort, I gained a shared, always up-to-date calendar, providing both an agenda of upcoming events and a searchable record of what I'd done and when; an always-connected email client, providing both an easy means of keeping in touch while on the road and, again, a searchable record of past communications; a shared, collated, searchable collection of notes concerning every relevant detail of my various clients' operations; a shared, &c., collection of contact information for all my various clients and industry contacts; and, last but not least, an always-connected administrative terminal with which to remotely solve even relatively subtle and complex problems, as and when necessary, for clients who had urgent needs and for whom I could not fit an in-person emergency visit into my schedule.
No doubt all of this makes it sound as though I was a preposterously busy person, and for several years that's exactly what I was. My smartphone enabled me to streamline my efforts in a fashion which I had hardly imagined possible before I first laid hands on the means of so doing, to the extent that it made me able to get more work done, with less effort, than I had been able to accomplish before. If there is any purpose at all behind our species' longstanding habit of building interesting trinkets and gewgaws, "more work with less effort" is certainly the very soul of that purpose.
I do, though, like your "only frivolous people with frivolous purposes would ever have a smartphone" argument, though. I think that's the most sensible thing anyone has said in this whole thread. Certainly it is in no way redolent of, for example, someone who bravely defies his ignorance in order to declaim at length on a subject of which he has absolutely no relevant experience whatsoever.