OP, lots of comments here, but I will just state my experience.
I got my sister's 3GS in December 2011 as a Christmas present. It was not my primary phone and I am not on AT&T, but my point in getting it from here was exactly the same as yours.
At the time it was iOS 5. And it was a very good introduction to iOS. I however, had to find workarounds for using the device as a phone (a secondary phone). Talkatone allowed me to use a GV number to actually make calls and text without having to force anyone else to use an iPhone or an iPhone app. iMessages covered me as well.
iOS 6 came out and I also got my iPhone 5 at the time. But the 3GS had prepared me for what I could expect. Now, the 3GS can only run iOS 6. That'll make it two versions out of date when iOS 8 releases. However, being on iOS 7 after trying very hard to stay on iOS 6, other than a few things, it is my own personal opinion that iOS 7 sucks eggs. iOS 8 may be better.
In any case, from a basics perspective, not much has changed over iOS 5, 6 and 7. Sure, new features have been added, but the basics are the same.
It's my opinion that when you upgrade you may appreciate the additional features after having learned the basics. It also might have you appreciating the simplicity of the basics as well.
I get where a lot of commenters here in this thread are stating that the 3GS is not good for todays modern times, but really, if all you are doing is basic stuff, you don't need things like Airdrop, Touch ID, so on and so on. Calls, texts and browsing will still work on a 3GS just fine. In fact, that 3GS I had, I gave it to a friend who needed a phone a few months ago. He's using it as his primary phone and it's worked flawlessly for him since.
Just my two cents.