I just came back to a question of mine and tried to type two short comments on the two helpful responses. I had a bad user experience, from my perspective, when I could not type the second one in because 15 seconds had not passed.
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The misuse of comments starts with users who post endless "me too", "I agree", and other such banal chit-chat. Having said that, if you really did post two consecutive, useful comments in 15 seconds, that is an unusual edge case.
Stack Exchange is a wide-open and largely anonymous public forum. As such, one of our best defense against wide-spread misuse, spam, and overt vandalism is to throttle EVERY activity users can perform to a reasonable, human level.
I believe that any possible inconvenience you experienced by having to wait a few extra seconds to post another comment is trumped by the damage that someone could cause if they raced through the system posting:
F YOU GUYS! – Robert Cartaino♦ 15 seconds ago