ORM's tend to lock one into a particular architecture. SQL is more standardized across platforms. It's my opinion that ORM's should assist with SQL generation or usage rather than outright hide it. ORM's job should be to reduce the grunt-work of preparing typical SQL, not wrapping it fully so you can ignore it.

You can take your SQL knowledge to another platform, but less so with say Entity Framework or LINQ. Don't get locked in. Plus, if the ORM doesn't do your query correctly or as intended, someone will need to understand SQL anyhow to debug it.