What are the arguments for "reviewers see each other's identities" conference reviews?
Historical reasons: Many conferences have switched to online PC discussions only recently, after having had physical PC meetings before. Since anonymity is not an option at physical PC meetings, a non-anonymous online discussion is closer to the experience of physical PC meetings.

Incentives for PC members. PC members do a lot of work without monetary compensation. There are some reputation-related incentives offered by a non-anonymous online discussion:


Junior PC members can "make themselves a name".
Senior PC members can prove that they are still good citizens of their community.
All PC members can get an impression of whom to invite and not to invite as reviewers for their own conferences and journals.