Fair enough -- I'm fine with using this as a springboard to continue with discussion, but I originally inferred that you had misinterpreted my point. Now that I understand where you're coming from I won't try to explain my own point further.
Part of the problem is that I really don't know how much work this would be, nor do I have any idea about how much of an impact it is. I suspect that it's a fairly simple change, especially if there's already information about the last modified time being recorded. If it's a one line alteration to a node template, for instance, I can't see a reason not to implement the change -- it may not provide tons of benefit, but if it's trivial to implement, everyone wins (not obtrusive enough to be detrimental to the people who don't care about the modification time). Obviously if it's a difficult change or would impact site speed considerably, drop the idea in the bit bucket and we can all continue on our merry way.
The best idea I have to deal with harmful updates would be a built in revision history, with difference comparisons -- akin to what most wikis have in place already. But I think that the quantity of work involved and the impact to the site would both be rather large (note: I'm understating how much work I think it would be). I personally would not be a fan of a node lock, I like being able to edit the things I write (particularly for grammar and spelling, without actual content changing).