Database specialists usually end up with very specific knowledge of a particular platform, for example Oracle where there are endless ways to tweak for performance, or replication or whatever. Modeling with RDBMS is usually very straightforward and ends up being only a minuscule fraction of what you do day to day.
If you are looking for someone to do schema design, that damn well better be in the job description given to candidates before they apply, and your software engineers damn well better be compensated for the extra required skill.
I tend to ask questions more about database internals, but it's a crucial job for an engineer to understand how to get performance and integrity for their queries.
Sometimes times you may get world-class DBA/DBE (my current employer is one of those places) who can do an engineer's job (in terms of tuning the queries, etc...) but that is really an engineer's job.