DB_PRIVATE enables a few optimizations by making the assumption that only one process will access the db4 database. Notably, this flag enables db4 to use pthreads-style mutex locking rather than heavy, operating-system-provided flock and shared memory. Ref: DB_ENV->open documentation.
The general motivation is that db4 databases can be safely accessed in parallel with bitcoin client, assuming (a) DB_PRIVATE is removed and (b) bitcoin properly uses db4 transactions. db4 transactions may even be employed to wrap around non-db4 data such as blk0001.dat, if the code is properly architected.