I asked a long-time EQ1-player friend a similar question about resists after seeing so many mods that feature them. His answer was that it was an easy way to check for debuffs in combat situations.
Personally, I guess I have a fairly large portion of my view taken up by the UI…but most of it is what I'd consider necessary. Long target/implied target windows so I can tell which arts to use when playing my swashbuckler, or how damaged the MT is when playing my inquisitor. Double XP bars for when I'm playing my summoner/alchemist. A chat window for generic spatial, and another for tells/group/raid/guild (although I'm no longer in a guild) but both of those autohide. A slightly extended player window to include HP/PP percentages so I can tell when to cut and run, and which combat art/spells I've got the juice left to use. It's only really with my swashbuckler that I
need to be able to see a decent amount of rendered space to line up those rear attacks…other than that I fall back on my MUD instincts and go by the text as much as the graphics.
For those curious:
my UI