Making sure you're not being stupid is very difficult. Generally I find that stupid people lack the brain power to know when they're being stupid, so the first warning they get is when things go horribly wrong. And being really smart is no help when you're in other people's power, which invariably happens when you travel. I wouldn't worry about it, it's out of your control.
So, to answer the question: what do you normally use a phone for and how do you define phone in any case?
A calls-and-text phone is useful for connecting with people for a hundred different reasons. When you land in a new town at midnight and can't find your hostel, a phone call is a life-saver. When you meet a beautiful and/or fascinating person as you go about your business, and they ask for your phone number so they can arrange to meet you later, what do you want to reply? What if you're on the road to connect with a dive trip or safari, and you get delayed? How do you let them know to wait for you?
One thing that many travellers get caught out with is the fact that services like gmail and hotmail will lock you out when you travel overseas. They want to send some confirmation code to your phone, or other service that you also can't get into. So you might need your original SIM card anyway. But if you're connecting via wifi on your normal device, this doesn't usually happen.
So you may well end up carrying your smartphone anyway, just to use whatever online services you usually use. Booking hostels and trips is a lot easier if you can get online, and relying on internet cafes and the single solitary computer in the hostel is not a great idea. Another reason to keep that smartphone!
But when you travel you frequently find yourself in unusual situations. Dropping that expensive toy out of a moving bus into an ankle-deep puddle in the midst of an "is this my stop" moment is guaranteed to ruin your day. Not to mention the risk of theft or loss. You might want to keep the valuable phone tucked away somewhere safer, and carry a throw-away for general use.
I use my very cheap stupid-phone as an alarm clock, means of communication, music player, and stuff it into my underpants in vibrate mode on long journeys. It only needs charging once a week, and my solid gold Vladimir Putin iPhone never gets stolen or damaged.
