Funny enough, that's one of my phone screen questions. I'd say anyone that has to do more than consume data from SQL Server really has to know this. It's one thing if they're just moving data around. However, they're not simply going to disable/enable indexes, there's a good chance that they'll occasionally need to drop & recreate them. Not knowing what a clustered index is and the implications it has on non-clustered indexes is a sign of a junior person. However, that's my opinion, not hard fact.