I think it’s a good idea to upgrade to a smartphone if you can get one cheap or for free (which you can, with a contract). Even if you’re a casual phone-user, there’s still stuff like GPS and weather apps that are extremely handy for anybody. QWERTY keypads (or touchscreens!) are great, too. And it’s nice to have a phone that can play decent-sized games or browse the web, if you’re ever bored on a train or at work.
Even if you decide to stay with a dumbphone, I think you’re overpaying for what you’re using. If you aren’t still in a contract, you need to shop around. I’m paying about $38 a month for my current plan, which is 300 minutes + I forget how many texts. 300 or 500? Doesn’t matter, since I never use all of my texts or minutes. And I’m DAMN lucky to have that 300-minute plan, it’s an old one of T-mobile’s that I got grandfathered in. They don’t offer it anymore. It’s like 500 minutes, 1000, or unlimited now.
You could go pre-paid, if you truly don’t use your phone much. But I’m not educated enough to wax about pre-paid options. Also I don’t think there IS such a thing as a pre-paid smartphone. But I could be wrong.
If you decide you want a smartphone, and you’re willing to enter into a 2-year contract, you can get a free smartphone through pretty much any carrier, I think–not the latest model, but there still seem to be some decent options. I don’t know which has the best prices at the moment, though. T-mobile has been good to me, but since they’re being bought out by AT&T, I’m planning to let my contract expire. Then I’ll shop around for a free smartphone+2yr contract somewhere else.
If you still want a smartphone but DON’T want a contract, you could get one through Virgin Mobile. Their unlimited data+minutes+text = $55 a month, according to their website (that’s super cheap! T-mobile’s comparable smartphone plan appears to be $80). Of course, since there’s no contract, you have to pay upfront for the phone. But their lowest end smartphone is only $90 right now. It’s not the best smartphone out there, but it’s probably still better than yours or mine 
Virgin Mobile probably has the best setup for casual phone users (which you, like me, definitely sound like). No ETFs to worry about since there’s no contract. And I know my uncle went with Virgin phones for himself and my cousin over a year ago, and they both really seem to like them. I may quite possibly go with Virgin when my T-mobile contract expires (especially if their “unlimited everything” plan stays so cheap compared to the big dogs).