From Bugzilla Helper: Using gdm to login doesn't touch X. Logging out from KDE or Gnome, on the contrary, restarts the entire X. I'd assume that this behavior is suboptimal and wrong. X is big and should be preserved in memory. This causes huge memory stress for the system and the blinking is not very healthy for the monitor. Reproducible: Always Steps to Reproduce: 1. Use graphical login 2. Login, then logout... 3. Watch for pid of X... Actual Results: X gets restarted - its PID changes and the screen is blinking. Expected Results: Preserve X from restarting and video mode from changing back and forth.
There's nothing to be gained by keeping X in memory, and no monitor that I know suffers adverse consequences from swapping video modes (since all PCs do this every time they're restarted, and many PCs are power-cycled several times a day for years with no damage to the monitor). The restart lets new logins use a "clean slate," which may reduce memory usage, probably increases stability a bit, and keeps users from playing dirty tricks on each other.
Of all your arguments I only agree with "clean state". As for dirty tricks, what could be worse than restarting X on a system with multiple remote X terminals? Plus, log out does take a few seconds to comlete even on a modern system with Athlon 750 MHz. I suppose 90% of this time is restarting X. RedHat has two options - graphical and text login. I think they must have a meaning besides esthetic one. Text mode should be used to achive "clean state" and prevent dirty tricks. Graphical mode should be used when X needs uninterrupted execution as on systems with multiple X users.