These are probably the most important factors for you. When I got my iPhone the purpose was, first and foremost, to have my MP3 player (something I use ALL the time) and my phone (something society - OK, my wife - expects me to always carry) on the same device. I use the thing for far more than that now, but I suspect in your case, since you’re already working from home, a smart phone isn’t so necessary. It’d be fantastic to have a smart phone when I travel, but as you say, you need to be on a local plan to use it, so then - when I most want it - the features aren’t available.
What do you need a camera for? If you use one for random shots day by day, then a smart phone is great, in a “the best camera is the one you have with you” sense; if you just use an SLR when you’re travelling, then an iPhone won’t help. When do you need a GPS? If it’s occasionally to get your bearings, a smart phone is good; if you’re trekking and route mapping in Alaska, you want a dedicated GPS device.
Having the internet always with me is cool (but seldom essential) for numerous reasons (alidarbac above has listed most of them), but I suspect that for your lifestyle, when you’re out you don’t particularly want the internet always with you; when you travel and are doing research, it’d probably be great, but as far as I know you can’t yet just pick up a temporary smart phone plan like you can a near-disposable SIM card. When/ if that happens, I think a smartphone would be fantastic for you, but until then, it’s likely not necessary.
The fact that I can carry a half-assed camera, an adequate GPS, a great MP3 player, a web browser, several books worth of Chinese flashcards and dictionaries, a reasonable e-reader, and oh yeah a phone, in one pocket is something that’s [strike]essential[/strike] [strike]important[/strike] useful to me every day. If you don’t feel like you need that, great, you’re probably financially more sensible than I am. Basically I wouldn’t bother until you see someone doing something with a smartphone and you think that’d be very useful to you, rather than just fun.
Another way to put it: what goes in your bag when you go out on a normal day? If it’s just a phone, forget smartphones. If it’s a phone and an MP3 player, consider it. A phone, an MP3 player, and a Kindle? Think harder. A phone, an MP3 player, a Kindle, a camera, a GPS, a stack of Chinese flash cards, several board games, and a book of crossword puzzles? Get a smartphone, but first get professional help.