The second part would actually be rather annoying to both tanks and dps.
If the tank is doing 5man/trash target switching is rather frequent so the raid mark will jump around alot and the spam will drive everyone crazy.
Tanks if they bother to mark, mark what needs to die first and then proceed to do whatever it takes to gain the attention of all uncontrolled mobs (this often requires target switching for interrupts/stuns/spreading hate around etc).
Well behaved dps either "nuke the skull" (if a mark is up) or assist the tank (if no mark is up).
In raids trash are handled pretty much the same way so the above applies and bossfights have specific assignments so auto-marking the tank's target is not really helpful.
Finally, everyone can mark in parties nowadays there's no requirement to be party leader.
