i have done a bit of work in this area.
Blizzard provides castbars on nameplates only for the current target (and a couple of addons try to provide castbars on nameplates for arena opponents). just enabling standard enemy nameplates probably won't help in PvE boss encounters, not by itself.
as for the event stream, you get one set of events (the UNIT_SPELLCAST_* events) for units with "permanent" non-target unitids (definitely groupmembers, and perhaps scripted-encounter bosses and arena opponents, haven't tested for that), plus the current target. on top of this, the combat log "indiscriminately" covers everything within "range" (the COMBAT_LOG_EVENT_UNFILTERED event and its endless variety of subtypes), including units with "permanent" unitids (effectively duplicating most of the UNIT_SPELLCAST_* events, with variations in the information supplied).
there is a broad issue, in PvE, with associating combat log events (which contain unit names and unit GUIDs, but not unitids, which in any event can be transient and may/may not exist at any given moment) with specific units in the environment. in PvP, PC unit names (i.e. player names, differentiated by realm) can generally be assumed to be unique (although you will run into the occasional hunter who has a pet named the same as the player, which can confuse "targeting aid" addons). but in PvE, there can often be several attackable NPCs nearby with the same unit name and level, and differentiating them without a target unitid is basically impossible.
Blizzard has recently added "permanent" non-target unitids for bosses in scripted instance encounters (which i have not yet tested), but ambient trash and spawns will not benefit from these unitids.
the only workaround to this i have found are raid target (icon) assignments... but even that only works if you are interested in being able to identify a graphical element like a nameplate. raid target assignments are not reported in the event stream for spell-cast activity, and the Blizzard API requires a unitid...
in short, this is a difficult problem.
AloftCastWarning attempts to solve the problem, within these various limitations, for nameplates. i would be happy to talk about the hoops i had to jump through, just send a PM my way.
AloftCastWarning has not yet been enhanced to incorporate Blizzard's new scripted-encounter boss unitids, but it should be capable of using them (and i should be able to test it solo in low-level dungeons). as well, most instance bosses have unique names. i hope to get to this fairly soon.
hope that helps.