I'm not sure I parsed your comment correctly, but it's trivial to modify the logging format to add things like response time. "%D" for Apache will add response time.
This is orthogonal, anyways, because if you can add more information with a label, you can add it without a label too.
Edit: I apparently missed your point. The issue about being able to modify the logging format at arbitrary times without breaking tools seems to be the main concern. Do people typically change their Apache logging format from time to time?
Yes, I think. There is no doubt about increasing the importance analysing of access logs.
Some people, including me, have experiences to customise parsers after expanding combined format with several fields. It's annoying to re-write regular expressions for parsers of tools we use when changing log format and to remember the meaning and order of expanded fields.
With LTSV, parsers does not need to be modified in that cases. Easy to expand logs and easy to process them with labels.
This is orthogonal, anyways, because if you can add more information with a label, you can add it without a label too.
Edit: I apparently missed your point. The issue about being able to modify the logging format at arbitrary times without breaking tools seems to be the main concern. Do people typically change their Apache logging format from time to time?