Yes, I agree that this would be useful and we plan to add this to TestRail. In fact, we plan to use both entered test times and estimates to calculate forecasts for runs, plans and milestones. We expect that we will be able to generate extremely accurate forecasts with this as more and more test times are entered. Of course we will also display how long a test run took to execute.
In the meantime, you can export run data to XML files and calculate reports yourself (the XML files contain the full test time data). I know this isn’t a great workaround, but might be a solution until we add this directly to TestRail.