Well, what you say is true, but the truth of the matter is a lot of people don't find pressing the power or home button on a phone a big problem, as evidenced by the huge decline in watch-wearing consumers. I certainly don't mind that. EVERYONE I knew used to wear a watch, but I'd be surprised if it was even 25% of my friends and family do now.
I'm guessing it is because you don't have to remember to bring a watch, and you don't have to spend money to buy a watch. And for mechanical watches it also means you don't have to adjust the date or the time. In that context, I personally find having only the smartphone more convenient than have both a phone and a watch.
What's also telling from your comment is that it sounds like you'd also think Smart watches suck, at least as currently implemented. You want something extremely convenient and out of the way, that you just glance down quickly to see the time and date. That is not the purpose and design of a smart watch in 2013.