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.
The article http://blog.vuscode.com/malovicn/archive/2007/08/05/releasing-the-build.aspx compares different options for debug and release and confirms that in 2007 pdbonly was the default for release configuration of visual studio
The article Include .pdb files in Web Application Publish for Release mode (VS2012) wasn’t applicable for us, but may be useful for someone else.