I don't personally, except on rare occasions when I'm reporting certain bugs or quirks, but some people will open multiple tabs with different threads.
In any case, while this may be intentional, I personally don't think it makes sense. For example, you go into a thread at 9:30 AM and it takes you 30 minutes to post. After you post at 10:00 AM, you have to leave for work, so you just leave without going to another page or anything. Your last activity will be shown as 9:30 AM, and that would really be confusing because people will see you posted at 10:00 AM, but wonder how your last activity was 9:30 AM.
On vBulletin, your activity updates when making a post with quick reply, as well as when doing other similar things, such as using quick edit. I think I'll have a look and see if a suggestion already exists for this.