>After doing this for a while, you can use these relative effort estimates to project completion dates for new projects.

This doesn't work. The problem is that you get periods where the work is relatively easy overall so 5 points ends up being tasks that take 2 days. Then you pick up new difficult features and estimate everything relative to all of the new hard tasks and you end up with 5 points meaning 5 days.

Sure you can gauge relative difficulty of tasks between each other, but in a regular developer's day-to-day life the range should really be 1-100 or so rather than making stuff fit into 12 points or less.