You are right about reuse across multiple internal systems not being strongly discouraged. Where it does discourage reuse is across multiple systems, and with

You are right about reuse across multiple internal systems not being strongly discouraged. Where it does discourage reuse is across multiple systems, and with websites. Making me change my corporate password every 90 days is an effective way to ensure that I don't use my current password across a large number of websites. Maybe I'd go to the effort of making my gmail password the same as my corporate password. The password on that random news site account I forgot about? No ways. It's not a panacea, but password expiry does effectively limit the spread of passwords in many cases.