1. Waste of space.
Comments themselves can go into one thred. We shouldn't have to scroll a whole page just to get onto a different topic
If there is already an active thread on the subject open, why not ammend it? What if i posted another topic on multiple threads in this same forum. What would the point of that be? All i'd be doing is pissing off a lot fo people, and calling (unwanted) attention to the fact that im ignorant and can't post in a topic concernign my same thoughts.
Locking usually only occurs when the topic becomes too off-topic to even know what is going on anymore, or users begin trashing/badmouthing other users or ideas.