I  think  they should be on by default. I wouldn't go so far as to try to make it difficult for people to bypass the default if they want to,. Having something like a visual cue - e.g. field turns red when going past the limits might be nice. I once saw someone complain that the limits must have been based on an old, infirm person, but I suppose that's a matter of opinion as to what they should be. If you're doing something like a machine as markschum mentions, then it's an easier decision. If we had really sophisticated figures with variable properties like weight and age etc, it would be nice to have dynamic limits that adjusted to fit those characteristics.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken