Personally, I think that it depends on the tone of the thread. You can post a different slant on a topic already under discussion and have it be inconsistent with what has gone before, completely losing the intent of the thread starter.
Sometimes that works, because the original idea had run its course. Other times it sends the thread off on a completely different tangent. That can be frustrating for the person who asked the original question, because they haven't really gotten the solution they were looking for before the thread takes off somewhere else.
Then too, sometimes it's a case of timeliness. There are topics discussed 5 years ago that may no longer be valid today. Restaurants come and go, websites change, retailers add and subtract product lines, etc.
Another negative: If I post a new comment on a 5 year old 50 page thread, then a new member wants to get some idea of what has inspired my post, he may have to try and sift through most of those 50 pages to get where I'm coming from. If I start it as a new thread, he's in on the ground floor, right along with me.
With this said, I don't advocate posting a new thread every time one has a thought. Use common sense and at least use the search function and see if there is an appropriate thread that would be suitable for what you want to bring up.
One other pet peeve of mine, and one that is enforced on some other sites I participate in, is the multi-quote button. I sometimes wonder if the person posting really doesn't understand its function, or if they are just ignoring it to pad their post count. It can potentially save a lot of pages on a thread to keep multiple quotes to a single post rather than a single member making 5 consecutive quoted posts.
I use the multi button most of the time, even if I end up only quoting one post, because that way as I read a thread and feel I have a comment to make on a post, I just click the multi button then read on, then click another one, etc. Then at the end, I just click Reply and all of the posts I want to quote, whether one post or five, come up in one thread so I can answer them in order. It's just more efficient for me and makes the thread less cumbersome to read.