There is nothing to gain from explicitly "ejecting" drives before logging pout, shutting down, or restarting in a normal fashion. MacOS X has a subsystem called "Disk Arbitration" (primarily the system daemon process "diskarbitrationd") which mounts drives other than the boot device when a user logs in and unmounts (a.k.a. "ejects") them when the user logs out. Because it operates outside of the context of a user login session, diskarbitrationd is able to assure that processes which are not directly managed by a user (Spotlight indexing, Time Machine, other users' login sessions, etc.) are dealt with correctly when unmounting devices. It also checks for (& usually can quietly repair) filesystem corruption issues before mounting drives. At a deeper level, the OS also has a daemon (update) which flushes the filesystem cache to disk every 30 seconds and a carefully scripted process for shutting down (i.e. the first half of restarting) which assures that all filesystem devices are properly synched and unmounted before resetting or powering down the system. MacOS X is unique in this respect only in the fine details: all significant OS's have matured beyond the need to have users manually assure that drives aren't unmounted in a "dirty" state.
The possible exceptions would be if you do something outside of the operating system to halt or reset the system, such as disconnecting the main power or forcing a hardware reset by a long-press of the power switch. Both could cause filesystem corruption, but no one should be taking such steps except in rare and severe cases where one likely won't have the luxury of being able to manually eject drives. (e.g. smoke, flames, kernel panics, etc.)
