It would very informative to have an endtime of when the query execution completed next to the duration of the query in SQL Server Management Studio.

One example,
You execute a long running query, leave the window and comeback "later". You can see that the query completed in "00:10:00" but not what time it actually ended so that you can send an accurate email saying "all the records have been updated as of time X"
This would help in the case that you execute a long running query and come back to it later seeing that it finished in "00:10:00" but not knowing when it actually finished.

Another example,
Say you are running a purge query in one query window and a SELECT COUNT(*) in another query to see how many more records need to be purged. You leave that window and come back to it "later", well you know how many records were left from the last SELECT COUNT(*) but you don't know what time that was executed and therefore when you rerun your query and get the new count of how many records are left you can calcuate how many records were purged since "earlier" but not how long it took to purge them since you didn't have a timestamp of from "earlier".