For a long time I believed that PDB as a part of debugging information should not be included in production deployment. Recently my colleague suggested to copy them to simplify exception investigations.
These files will not be exposed to the public if kept in the right places (website\bin).
So we decided to deploy the PDBs into production.
However we found that PDBs were generated not for all DLLs. After some analysis we believe that MS changed default settings starting from VS 2008 (or may be since VS 2005) and make generation of PDB-only even for release mode. This is why older projects had generation of PDBs disabled.
/optimize+ /debug:pdbonly (release configuration of visual studio)