I don't see how it can be called bloated, beyond the usual "ZOMG WINDOWS USES XXXXMB OF RAM LOLZZ" stuff.
AD isn't just LDAP, it's a central store for everything management. Yes it holds your authentication details, but it also holds settings for (assuming you use the MS products) DHCP, DNS, Mail, etc. Want a new DC to avoid SPOF? Install Windows, install the role, promote. All the settings are copied down automatically and you're redundant. You've also got the concept of sites, which certain domain controllers handle, so workstations know where to find their local DC. Also, subdomains, so you can hand off sections of the environment to other people - think company divisions (dev.corp.net) or even countries (us.corp.net). It's all GUI and PowerShell controllable these days too.
As for replication failures, yes they happen, as it will with anything that depends on replication. Disagree with them being common though - I've seen one in just over 15 years. We deleted the VM, made a new one on a new name, ran a clean up tool and carried on.