It's situation based. In some non-task focused discussions topic/subject drift can be a VERY useful tool. In a needing to be structured situation it can frankly wreck historical review value.Trying to use a list archive for problem solving Vs nominal chat is indeed going to change one's feelings on the concept. SO- if we are flogging a political or social dead horse it's less deadly to drift topic/subject lines.

IF we are working on something of future search value then managing subject line to content relationship IS important.
And if not? The dangling bit intentional of course..