Sure, you can smoke in anything really, I've smoked food in something as simple as a wok before.
Still having a separate fire box & being able to control the heat consistently will always give you a better result in the end.
You can control heat consistently with the Weber kettle lol. Seriously, don't write it off as a smoker.. there are lots of people knocking out amazing smoked meats with the Weber kettles. I can guarantee that whatever I can do with my kamado, or Bananas can do with his pellet grill, you can do with your kettle.
Offset smokers are great if you WANT to spend your day tending the fire and working it. They're definitely not necessary though.. The only real advantage they tend to have is cooking space, which is why they are what is used in a commercial and catering context. For the average home user, they're unnecessary (but still a great option if they're what you want)